stm32mp1: change STGEN clock source to HSE

No more use static frequency HSI = 64MHz for STGEN clock
but HSE (with higher accurency) by default.

Need to remove CONFIG_SYS_HZ_CLOCK as arch timer frequency
is provided at boot by BootRom and cp15 cntfrq and modified
during clock tree initialization if needed.

When HSI is no more used by any device, this internal
oscillator can be switched off to reduce consumption.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

arch/arm/dts/stm32mp157c-ed1-u-boot.dtsi

@@ -76,6 +76,7 @@
 	st,pkcs = <
 		CLK_CKPER_DISABLED
 		CLK_SDMMC12_PLL3R
+		CLK_STGEN_HSE
 		CLK_I2C46_PCLK5
 		CLK_I2C12_PCLK1
 		CLK_SDMMC3_PLL3R

include/configs/stm32mp1.h

@@ -18,7 +18,6 @@
  */
 #define CONFIG_SYS_HZ				1000
 #define CONFIG_SYS_ARCH_TIMER
-#define CONFIG_SYS_HZ_CLOCK			64000000
 
 /*
  * malloc() pool size