ARM: dts: stm32: add usart2, usart3 and uart7 pins in stm32mp15-pinctrl

Adds usart2_pins_c, usart3_pins_b, usart3_pins_c and uart7_pins_c pins
configurations in stm32mp15-pinctrl.
- usart2_pins_c pins are connected to Bluetooth chip on dk2 board.
- usart3_pins_b pins are connected to GPIO expansion connector on evx board.
- usart3_pins_c pins are connected to GPIO expansion connector on dkx board.
- uart7_pins_c pins are connected to Arduino Uno connector on dkx board.

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>