ARM: imx_v4_v5_defconfig: Fix the selection of 32 bit flash support for iMX21ADS board

commit 1a96571d (Enable 32 bit flash support for iMX21ADS board) aimed to
enable 32 bit flash support, but all it did was to delete an unset option.

Fix this by enabling CONFIG_MTD_MAP_BANK_WIDTH_4 option.

Cc: Jaccon Bastiaansen <jaccon.bastiaansen@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>