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/board/st/stm32mp1/Kconfig b/board/st/stm32mp1/Kconfig
index 7d0d41b..96de415 100644
--- a/board/st/stm32mp1/Kconfig
+++ b/board/st/stm32mp1/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_ST_STM32MP15x
+if TARGET_ST_STM32MP15X
 
 config SYS_BOARD
 	default "stm32mp1"