configs: verdin-imx8mp: Fix wrong early malloc() heap size

Set, previously unset, CONFIG_SPL_SYS_MALLOC_F_LEN to 0x4000 whose
default value is 0x10000.
Early malloc() uses CRAM_S at 0x184000 (CFG_MALLOC_F_ADDR), this ram
area end at 0x188000.

Fixes: 2bc2f817 ("board: toradex: add verdin imx8m plus support")
Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>