uboot-imx

spl: Allow settings malloc_f base address

Allow configuring the begining of the malloc_f area in SPL.
This patch uses the same CONFIG_MALLOC_F_ADDR established by
the sandbox.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>