configs/stm32f429i-disco: Separate SPI4 from MTD init. I was trying to attach...

configs/stm32f429i-disco: Separate SPI4 from MTD init.  I was trying to attach a non-MTD peripheral to an STM32F429I Discovery Board's SPI4 port and was hitting compilation problems, since CONFIG_MTD and thus 'struct mtd_geometry_s' was not defined.  This patch separates SPI4 initialization logic from MTD initialization logic.