drivers: clk: agilex: Use real clock source frequency

drivers: clk: agilex: Use real clock source frequency

Update the ARMv8 generic timer frequency register (cntfrq_el0)
with the actual hardware timer frequency (COUNTER_FREQUENCY_REAL).

The generic timer frequency was set to 0x200000000 during boot clk
which needs to be set to 0x400000000 when transition from boot clk
to PLL clk.

This will ensure that subsequent timer operations are based on the
correct frequency, ensuring accurate timekeeping.

Signed-off-by: Alif Zakuan Yuslaimi <alif.zakuan.yuslaimi@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>