arm64: zynqmp: Describe 25MHz fixed clock for PL GEMs

Describe 25Mhz fixed oscilator which is providing clock for PL based
ethernet IPs. Physicially it is one chip but it is described as 2 fixed
clock to be aligned with other SOM versions which were using integrated
clock generators where clocks could be adjusted via i2c (si5332 chips).

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/c430aeacaa76d9f61ed3f874f721a33049f45eb9.1706514396.git.michal.simek@amd.com