Kconfig: Fix SYS_MALLOC_F_LEN for i.MX8MQ

i.MX8MQ use SYS_MALLOC_F_LEN 0x2000, not 0x10000. The OCRAM_S only has
32KB memory, use 0x10000 will make SPL not bootable.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>