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>
diff --git a/drivers/clk/clk_stm32mp1.c b/drivers/clk/clk_stm32mp1.c
index 0c0ef36..48c9514 100644
--- a/drivers/clk/clk_stm32mp1.c
+++ b/drivers/clk/clk_stm32mp1.c
@@ -540,6 +540,7 @@
 	STM32MP1_CLK_SET_CLR(RCC_MP_APB2ENSETR, 13, USART6_K, _UART6_SEL),
 
 	STM32MP1_CLK_SET_CLR_F(RCC_MP_APB3ENSETR, 13, VREF, _PCLK3),
+	STM32MP1_CLK_SET_CLR_F(RCC_MP_APB3ENSETR, 11, SYSCFG, _UNKNOWN_SEL),
 
 	STM32MP1_CLK_SET_CLR_F(RCC_MP_APB4ENSETR, 0, LTDC_PX, _PLL4_Q),
 	STM32MP1_CLK_SET_CLR_F(RCC_MP_APB4ENSETR, 4, DSI_PX, _PLL4_Q),