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/cpu.c b/arch/arm/mach-stm32mp/cpu.c
index 74d03fa..73d8b75 100644
--- a/arch/arm/mach-stm32mp/cpu.c
+++ b/arch/arm/mach-stm32mp/cpu.c
@@ -435,6 +435,10 @@
 		env_set("boot_device", "nand");
 		env_set("boot_instance", "0");
 		break;
+	case BOOT_FLASH_SPINAND:
+		env_set("boot_device", "spi-nand");
+		env_set("boot_instance", "0");
+		break;
 	case BOOT_FLASH_NOR:
 		env_set("boot_device", "nor");
 		env_set("boot_instance", "0");