ARM: dts: stm32h7-pinctrl: add _a suffix to u[s]art_pins phandles

ARM: dts: stm32h7-pinctrl: add _a suffix to u[s]art_pins phandles

Allow expanding possible configurations for the same peripheral,
consistent with the scheme adopted in Linux.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/r/20250427074404.3278732-2-dario.binacchi@amarulasolutions.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
[ upstream commit: 6a36dca4375fce51b627f5a985a79fc8b8bd7f55 ]

(cherry picked from commit 9a72c83f2e670087ae2d6dc54d2926f16c6762d0)