rockchip: rk3368: add sdram driver for U-Boot

Add sdram driver in U-Boot for get the correct sdram size from
sys_reg, so that U-Boot can co-work with Rockchip loader or SPL
to get different dram capability and then tell the kernel.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>