board: stm32mp1: support boot from spi-nand

Manage BOOT_FLASH_SPINAND, with boot_device="spi-nand"
and treat this value in bootcmd_stm32mp.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/arch/arm/mach-stm32mp/spl.c b/arch/arm/mach-stm32mp/spl.c
index 9fbc3ac..760fe45 100644
--- a/arch/arm/mach-stm32mp/spl.c
+++ b/arch/arm/mach-stm32mp/spl.c
@@ -39,6 +39,8 @@
 		return BOOT_DEVICE_NAND;
 	case BOOT_FLASH_NOR_QSPI:
 		return BOOT_DEVICE_SPI;
+	case BOOT_FLASH_SPINAND_1:
+		return BOOT_DEVICE_NONE; /* SPINAND not supported in SPL */
 	}
 
 	return BOOT_DEVICE_MMC1;