rockchip: rk3568: Fix alloc space exhausted in SPL

Current SYS_MALLOC_F_LEN of 0x2000 (8 KB) used in SPL is too small for
some RK3568 boards. SPL will print following during boot:

  alloc space exhausted

Increase the default SYS_MALLOC_F_LEN to 0x20000 (128 KB) to mitigate.

Fixes: 2a950e3b ("rockchip: Add rk3568 architecture core")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>