ARM: imx: Fix the removal of CONFIG_SPI option

Since 64546e9f ("ARM: imx_v6_v7_defconfig updates") and commit
0650f855 ("ARM: imx_v4_v5_defconfig: Select CONFIG_IMX_WEIM") CONFIG_SPI
selection was dropped by savedefconfig for imx_v4_v5_defconfig and
imx_v6_v7_defconfig.

In order to keep the same behaviour as previous kernel versions and avoid
regressions, let's add CONFIG_SPI option back.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>