Cleanup use of CONFIG_SYS_SRAM_BASE and CONFIG_SYS_SRAM_SIZE

Cleanup use of CONFIG_SYS_SRAM_BASE and CONFIG_SYS_SRAM_SIZE

Traditionally many boards used local definitions for SRAM base address
and size (like SRAM_BASE, SRAM_LEN and/or SRAM_SIZE), while the (now)
"official" names are CONFIG_SYS_SRAM_BASE and CONFIG_SYS_SRAM_SIZE.

The corresponding code in arch/powerpc/lib/board.c was board specific,
and has never actually been maintained well. Replace this by feature-
specific code and adapt the boards that actually use this.

NOTE: there is still a ton of boards using the old #defines, which
therefor contain incorrect values in bi_sramstart and bi_sramsize.

All respective board maintainers are requested to clean up their
respective configurations.  Thanks.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Josef Wagner <Wagner@Microsys.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Wolfgang Denk <wd@denx.de>