arm: mx6: cm-fx6: increase size of malloc area

Increase size of malloc area to make room for framebuffer
and other such big allocations.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>