imx8mq_phanbell: Remove custom CONFIG_SYS_MALLOC_F_LEN

Currently, the imx8mq_phanbell board fails to boot.

Remove the custom CONFIG_SYS_MALLOC_F_LEN setting in favor of the
generic one done via the main Kconfig.

Since commit b5989572 ("Kconfig: Fix SYS_MALLOC_F_LEN for i.MX8MQ")
the default SYS_MALLOC_F_LEN value for i.MX8MQ is 0x2000.

With such default value, the board boots again.

Signed-off-by: Fabio Estevam <festevam@denx.de>