clk: loongson2: Avoid hardcoding firmware name of the reference clock

Loongson-2K0300 requires a reference clock with a frequency different
from previous SoCs (120MHz v.s. 100MHz), thus hardcoding the firmware
name of the reference clock as ref_100m isn't a good idea.

This patch retrives the clock name of the reference clock dynamically
during probe, avoiding the hardcoded pdata structure and preparing for
support of future SoCs.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>