clocksource/drivers/arm_global_timer: Add auto-detection for initial prescaler values

am43xx has a clock tree where the global timer clock is an indirect child
of the CPU clock used for frequency scaling:

  dpll_mpu_ck -- CPU/cpufreq
        |
        v
  dpll_mpu_m2_ck -- divider
        |
        v
  mpu_periphclk -- fixed divider by 2 used for global timer

When CPU frequency changes, the global timer's clock notifier rejects
the change because the hardcoded prescaler (1 or 2) cannot accommodate
the frequency range across all CPU OPPs (300, 600, 720, 800, 1000 MHz).

Add platform-specific prescaler auto-detection to solve this issue:

- am43xx: prescaler = 50 (calculated as initial_freq/GCD of all OPP
  freqs) This allows the timer to work across all CPU frequencies after
  the fixed divider by 2. Tested on am4372-idk-evm.

- zynq-7000: prescaler = 2 (preserves previous Kconfig default)

- Other platforms: prescaler = 1 (previous default)

The Kconfig option now defaults to 0 (auto-detection) but can still
override the auto-detected value when set to a non-zero value,
preserving existing customization workflows.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Judith Mendez <jm@ti.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20250819-topic-am43-arm-global-timer-v6-16-v2-1-6d082e2a5161@baylibre.com