rockchip: add sdram_common for common functions

There are some functions like sdram_size_mb can be re-used for
different rockchip SoCs, just put them into common file.
Add board_get_usable_ram_top() for ram_top init base on
SDRAM_MAX_SIZE.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Added SDRAM_MAX_SIZE definition for RK3036:
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
fixup: 3036 fix for sdram_common