rockchip: Ensure memory size is available in RK3399 SPL

At present gd->ram_size is 0 in SPL, meaning that it is not possible to
enable the cache. Correct this by always populating the RAM size
correctly.

This increases code size by about 500 bytes in SPL, since it must call
the rather large rockchip_sdram_size() function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>