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>