clk: en7523: Add clock driver for Airoha EN7523 SoC

This driver only registers fixed rate clocks, since the clocks are fully
initialized by the boot loader and should not be changed later, according
to Airoha.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20220314084409.84394-3-nbd@nbd.name


Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>