ramips/rt305x: add initial support for Rt5350 SoC

Somehow detecting the RAM size in common/setup.c doesn't
work here, it always detects 64M and then crashes on devices
with less RAM.
Probably using MEMC_REG_SDRAM_CFG1 to know the RAM size is how
it could be, for now I use the mem=32M kernel parameter to get
stuff working.

Signed-off-by: Daniel Golle <dgolle@allnet.de>

SVN-Revision: 33381