clk: stm32mp1: add support of SYSCFG clock

clk: stm32mp1: add support of SYSCFG clock

Add the support of SYSCFG clock used by syscon driver
to prepare the clock management of STM32MP_SYSCON_SYSCFG.

This clock is already defined in kernel device tree,
stm32mp151.dtsi but not yet supported in the syscon driver:

syscfg: syscon@50020000 {
	compatible = "st,stm32mp157-syscfg", "syscon";
	reg = <0x50020000 0x400>;
	clocks = <&rcc SYSCFG>;
};

It is safe to support this clock in U-Boot driver with
RCC_MC_APB3ENSETR, Bit 11 SYSCFGEN: SYSCFG peripheral clocks
enable.

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