clk: stm32mp1: Add support for USART1 clock

Add USART1 clock parents and mux configuration. This allows
support for configuring the USART1 as the serial console in
SPL and U-Boot via device tree. Without this patch the SPL
with usart1 serial console enabled crashes because it can
not find the clock specified in the device tree for usart1.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>