configs: stm32mp13: add SPI-NAND UBI boot support

configs: stm32mp13: add SPI-NAND UBI boot support

The STM32MP13 default environment only handles MMC and serial/USB boot.
When TF-A reports BOOT_FLASH_SPINAND the boot_device variable is set to
'spi-nand' but bootcmd_stm32mp never redirects boot_targets to ubifs0,
so distro_bootcmd falls through to MMC/USB.

This change mirrors the STM32MP15 logic:
 - Add a BOOT_TARGET_UBIFS entry to BOOT_TARGET_DEVICES so that
   bootcmd_ubifs0 is defined (ubi part UBI; ubifsmount ubi0:boot).
 - Add the 'spi-nand' / 'nand' clause to bootcmd_stm32mp so that
   boot_targets is set to 'ubifs0' when booting from NAND.

Signed-off-by: Austin Shirley <austin@deadband.dev>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: uboot-stm32@st-md-mailman.stormreply.com