arm: mx6: gw_ventana: Define CONFIG_SYS_MALLOC_F_LEN

arm: mx6: gw_ventana: Define CONFIG_SYS_MALLOC_F_LEN

This board uses setup_i2c() in SPL. The setup_i2c() function internally
calls gpio_request(), which in turn internally calls strdup(). The strdup()
requires a running mallocator, so this patch makes the mallocator available.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Sean Cross <xobs@kosagi.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>