configs: icore: Fix U-Boot proper loading from nand

SPL on Engicam i.Core M6 boards enabled DM, so it would require some
malloc() pool before relocation in order to load U-Boot proper properly.

So, enable SPL malloc() pool of 0x2000 size similarly like what we have
used for icore mmc defconfigs.

Signed-off-by: Shyam Saini <shyam.saini@amarulasolutions.com>