rockchip: Explicitly set CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN

rockchip: Explicitly set CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN

Now that u-boot relocates the malloc area in SPL to SDRAM, with the
malloc area sitting below the SPL_STACK_R_ADDR the
SPL_STACK_R_MALLOC_SIMPLE_LEN needs to be set explicitly for
rockchip as its SPL_STACK_R_ADDR (512kb) is smaller then
STACK_R_MALLOC_SIMPLE_LEN (1Mb).

Using the same value as SYS_MALLOC_F_LEN (8kb) is enough to load u-boot
from SD card.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>