board_init: Convert CONFIG_MALLOC_F_ADDR to Kconfig

Convert CONFIG_MALLOC_F_ADDR to Kconfig.

The CL IoT gate board used MALLOC_F_ADDR 0x912000 on i.MX8M Mini unlike
all the other platforms which use 0x930000 . This has been aligned with
the other i.MX8M Mini platforms.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Kirill Kapranov <kirill.kapranov@compulab.co.il>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Peter Robinson <pbrobinson@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Uri Mashiach <uri.mashiach@compulab.co.il>
Cc: Valentin Raevsky <valentin@compulab.co.il>
Cc: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>