Revert "u-boot: set DDR3 clock back to 303MHz for boneblack"

Timings do work, failure was PEBKAC

This reverts commit 241bf185cb037ec26a18fdb983681f827d988e14.
linux-mainline 3.8: update to 3.8.7

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>