ARM: dts: stm32: fix ltdc pinctrl on microdev2.0-of7

It prevents the following warning:

 pin-controller@50002000: 'ltdc' does not match any of the regexes:
'-[0-9]*$', '^gpio@[0-9a-f]*$', 'pinctrl-[0-9]+'

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