phy: phy-snps-eusb2: refactor reference clock init

phy: phy-snps-eusb2: refactor reference clock init

Instead of matching frequencies with a switch and case, introduce
a table-based lookup. This improves readability, reduces redundancy,
and makes it easier to extend support for additional frequencies in
the future.

Signed-off-by: Ivaylo Ivanov <ivo.ivanov.ivanov1@gmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250504144527.1723980-9-ivo.ivanov.ivanov1@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>