rdn2: fix refclk clock speed

The refclk clock speed is incorrectly set to 100MHz for RD-N2 platform
configs 0, 1 and 2. It should be 125MHz instead of 100MHz. Fix this
accordingly.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I357feb55142f7ab332646aafbfef553cd40c7e1e