Fix uart7_pins_a comments to indicate UART7 pins instead of UART4 pins.

Fixes: bf4b5f37 ("ARM: dts: stm32: Add missing pinctrl definitions for STM32MP157")
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>