arm: Rename STM32MP15x

CONFIG options must not use lower-case letter. Convert this and related
ones to upper case.

Signed-off-by: Simon Glass <sjg@chromium.org
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/configs/stm32mp15_defconfig b/configs/stm32mp15_defconfig
index 4aff639..3302b30 100644
--- a/configs/stm32mp15_defconfig
+++ b/configs/stm32mp15_defconfig
@@ -9,7 +9,7 @@
 CONFIG_DDR_CACHEABLE_SIZE=0x8000000
 CONFIG_CMD_STM32KEY=y
 CONFIG_TYPEC_STUSB160X=y
-CONFIG_TARGET_ST_STM32MP15x=y
+CONFIG_TARGET_ST_STM32MP15X=y
 CONFIG_ENV_OFFSET_REDUND=0x940000
 CONFIG_CMD_STM32PROG=y
 # CONFIG_ARMV7_NONSEC is not set