How much motivation do berries restore?

While it will require more testing to know how much motivation berries will restore, it is stated on this article that a Razz Berry, Pinap Berry, and Nanab Berry restore the motivation in equal amounts. As you already mentioned, the Golden Razz Berry fully restores the motivation

However, it’s possible to replenish the motivation of one of your Pokemon. All you have to do is feed it berries. Any kind of berries are acceptable, so you can give it Razz Berries, Nanab Berries, or Pinap Berries. All three of those provide the Pokemon with the exact same amount of motivation, although a step above them is the Golden Razz Berry, as that one fully restores the motivation bar

Also, according to this article, the amount of motivation each berry restores decreases every time you use one to prevent a player from spamming berries to reach 100% motivation

If you want to fully restore your Pokemon’s motivation, though, you can’t just continuously feed it berries. That’s because when you feed a Pokemon multiple berries in a row, the berries provide the Pokemon with less and less motivation. This discourages players from simply visiting their gym over and over again and constantly keeping their Pokemon at high motivations so that no one can take the gym down. It also encourages cooperation, as although you can’t replenish a Pokemon by giving it many berries in a row, one of your teammates can give it a second berry which will provide just as much motivation as the first one you gave it.

According to Pokemon Go Hub:

Berries restore roughly 15-18% of your Pokémon CP on first use, similar to how much 1 lost battle reduces. After the first time you fed a Berry, diminishing returns start kicking in. Each successive Berry feeds for far less than the first one, roughly 50% less at first and that drop off continues!

The first berry will yield about 15-18% of a motivation gain, and about 50% less with each berry after that - making it more difficult to keep a Pokemon motivated. It should be noted though that this is for if you are feeding them berries rapidly. Niantic George on Reddit stated:

You can give Berries to any Pokémon on Gyms controlled by your team at any time. There are some significant diminishing returns if you feed the same Pokémon multiple Berries over a short period of time.

As noted on many sites (Pokemon Go Hub for example), the Golden Razz Berry will restore 100% motivation:

Pokémon GO Golden Razz Berry is a special variant of the commonly known Razz Berry. It can be used for two purposes: fully restoring a Gym Defender’s motivation meter and drastically increasing your chance to capture a wild Pokémon.