ARM: imx_v6_v7_defconfig: Enable SPI_GPIO

Some boards like imx7d-sdb provide SPIO functionality via GPIOs,
so enable the CONFIG_SPI_GPIO option.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>