spl: Introduce SPL_DM_SPI Kconfig define

This define indicates if DM_SPI shall be supported in SPL. This allows
proper operation of DM converted SPI drivers in SPL, which use
#if !CONFIG_IS_ENABLED(DM_SPI) to also support not yet DM/DTS converted
boards.

Signed-off-by: Lukasz Majewski <lukma@denx.de>