imx: defconfig: Enable CONFIG_SPL_FORCE_MMC_BOOT on display5

After the following commit:
commit 772b5572 ("imx: Introduce CONFIG_SPL_FORCE_MMC_BOOT to force MMC
boot on falcon mode")
it is possible to set the CONFIG_SPL_FORCE_MMC_BOOT flag, which allows
using MMC device as boot device regardless of the device used by Boot ROM
(FBL) as the first boot medium.

Display5 board needs this flag set to allow falcon boot from eMMC device.

Signed-off-by: Lukasz Majewski <lukma@denx.de>