stm32mp1: configs: Add CONFIG_SPL_SPI_FLASH_MTD

As SPI_FLASH_MTD is used in SPL and U-Boot proper, we enable both,
now that a separate option for SPL was introduced.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
[jagan: drop unrelated change]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>