Enable 32 bit flash support for iMX21ADS board

The kernel configuration for the i.MX21ADS board (imx_v4_v5_defconfig)
doesn't enable 32 bit flash support. But the i.MX21ADS specific code
adds a 32 bit flash device, resulting in a kernel that refuses to
boot.

Signed-off-by: Jaccon Bastiaansen <jaccon.bastiaansen@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>