spi: mpc8xx: Add GPIO dependency

Since commit 773ad4eb ("spi, mpc8xx: Add support for chipselect via
GPIO and fixups"), DM_GPIO is required for 8xx SPI.

Add the missing dependency to avoid build failures.

Fixes: 773ad4eb ("spi, mpc8xx: Add support for chipselect via GPIO and fixups")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>