powerpc/82xx: add SDRAM detection for km82xx

This patch adds SDRAM detection feature to km82xx boards.
To enable this feature, define CONFIG_SYS_SDRAM_LIST as the initializer
for an array of struct sdram_conf_s.
These structs will expose the bitfields within registers PSDMR and OR1 which
have to be different between configurations; common bitfields will be
defined, as usual, within CONFIG_SYS_PSDMR and CONFIG_SYS_OR1.
If CONFIG_SYS_SDRAM_LIST is not defined, then the usual behavior is retained.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>