spi: Kconfig: add dependency on CONFIG_MPC85xx for FSL_ESPI entry

The Freescale ESPI driver (CONFIG_FSL_ESPI) relies to the MPC85xx platform
(PowerPC) through arch/powerpc/include/asm/immap_85xx.h. The driver can't
compile on another architecture/platform.

Signed-off-by: Corentin Guillevic <corentin.guillevic@smile.fr>