sh: add common dram_init() function for all boards

Generic board support assumes a different method of specifying
DRAM size on board, also it can be shared among all boards, notably
only sh7763rdp board has a custom legacy dram_init(), however
the difference is only in printing some additional information,
this feature can be removed.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>