drivers: clock_control: stm32h5: boot_clock_assert_fix

When the image is chain-loaded, clocks may already by initialized.
The driver was lacking support for already configured HSE and
PLL sources. When CONFIG_ASSERT=y get_startup_frequency was
failing since it did not recognize these sources.

It's the same issue that was addressed in #58109 for stm32u5.

Signed-off-by: Kacper Dalach <dalachowsky@gmail.com>