configs: stm32mp1: stm32mp1: Increase SPL malloc() size

Since commit 03f1f78a ("spl: fit: Prefer a malloc()'d buffer for
loading images"), FIT images must be malloc()'d before being loaded.
The old size of 1 MiB is suitable for FIT images with u-boot and an
FDT, but something containing a linux kernel is almost sure to fail.

It's safe to extend malloc all the way to 0xc2000000, but no further.
Linux likes to be loaded at 0xc2000000, so we use that as our cutoff
point. This gives us 29 MiB of malloc() space, which suited for more
complex FIT images including several DTBs, kernel, and OP-TEE images.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>