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>