cmd: bdinfo: Move sram info prints to generic code

bi_sramstart and bi_sramsize are generic members of the bd_info structure,
so move the m68k/powerpc-specific prints to generic code. Also, print them
only if SRAM support is enabled via CONFIG_SYS_HAS_SRAM.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>