clk: starfive: Make _clk_get become a common helper function

clk: starfive: Make _clk_get become a common helper function

Introduce num_reg to store the number of clocks, this helps to make
_clk_get become a common helper function which called jh71x0_clk_get().
With this, it helps to simplify the code and extend the code in the
future.

Signed-off-by: Changhuang Liang <changhuang.liang@starfivetech.com>
Link: https://lore.kernel.org/r/20250114081300.36600-1-changhuang.liang@starfivetech.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>