ARM: dts: stm32: add reset support to uart nodes on stm32mp15x

STM32 serial IP can be reset via reset controller.
Add the support of reset to uart nodes on stm32mp15-u-boot.dtsi,
the ad-dons file for U-Boot.

This patch fix issues when previous UART configuration, for example
done in TF-A or ROM code, is not handled in U-Boot stm32 serial driver
init.

This reset property won't be not added in Linux kernel device tree
as this reset is not used in Linux STM32 serial driver.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>