ARM: dts: stm32: Fix User button on stm32mp135f-dk

This patch fixes the following dtbs_check warning on stm32mp135f-dk:
arch/arm/boot/dts/stm32mp135f-dk.dtb: gpio-keys: 'user-pa13' does not match any of the regexes: '^(button|event|key|switch|(button|event|key|switch)-[a-z0-9-]+|[a-z0-9-]+-(button|event|key|switch))$', 'pinctrl-[0-9]+'
From schema: Documentation/devicetree/bindings/input/gpio-keys.yaml

It renames user-pa13 node into button-user so that it matches gpio-keys
bindings.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>