imx8mm-cl-iot-gate: Enable cache in SPL

There is no reason for disabling I-cache and D-cache
in SPL.

Remove the unneeded CONFIG_SPL_SYS_ICACHE_OFF and
CONFIG_SPL_SYS_DCACHE_OFF options.

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