Here's one, attributed to Confucius
T’ung Jên
Life leads the thoughtful man on a path of many windings.
Now the course is checked, now it runs straight again.
Here winged thoughts may pour freely forth in words,
There the heavy burden of knowledge must be shut away in silence.
But when two people are at one in their inmost hearts,
They shatter even the strength of iron or of bronze.
And when two people understand each other in their inmost hearts,
Their words are sweet and strong, like the fragrance of orchids.
There are a few here:
http://www.blurry-joy-love-poems.com/friendshippoems.html
and lots here, but they are a bit sentimental
http://www.alighthouse.com/myeye.htm
and here
http://www.love-poems.me.uk/a_friendship_poems_index.htm