drivers: clock_control: stm32u5: fix chain-load with asserts

It is possible that stm32_clock_control_init function is started
when the image is chain-loaded and hardware clocks are already
initialized to some state (PLL1).

Currently due to an assert in get_startup_frequency function
(which will trigger k_fatal_halt) the boot will stop on the
early stage if CONFIG_ASSERT=y. This is reproducible for example
with standalone MCUboot and TF-M with MCUboot.

Fixed by adding support for HSE and PLL1 clock sources at
get_startup_frequency.

Signed-off-by: Denis Mingulov <denis@mingulov.com>