MLK-18654-4 imx8mm_evk: Enable SPL SDP for iMX8MM EVK board

MLK-18654-4 imx8mm_evk: Enable SPL SDP for iMX8MM EVK board

Enable the SPL SDP for iMX8MM EVK. So that when booting from USB serial
download mode, the SPL will enter SDP to download FIT image from USB host
(uuu tool) and boot into FIT image.

The SDP driver needs larger malloc pool size, so extend the SPL malloc pool
to 12KB and reduce the SPL BSS MAX to 4KB because BSS actual size is about 1KB.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit bd9cbf66f801ac930f7e62da9ed42653fd3dc7f9)
(cherry picked from commit f52234814d3d624aae36dd1db843f06d8ea6e348)