configs: evb-rk3229: Increase SPL_STACK_R_MALLOC_SIMPLE_LEN

An OP-TEE FIT image will fail to extract in SPL because the malloc stack
size is currently limited to 0x2000 for evb-rk3229 board.

In SPL we do not have to care about size limitations, since we are no
longer bound to SRAM limits after DRAM initialization has been done in TPL.

Use the default value for CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN in order
successfully unpack the FIT image.

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>