clk: airoha: Add econet EN751221 clock/reset support to en7523-scu

clk: airoha: Add econet EN751221 clock/reset support to en7523-scu

EcoNet EN751221 clock/reset driver is significantly similar to the
EN7523 / EN7581, however the EN751221 does not have a neat batch of clock
divider registers so there are fewer known clocks, and the frequency of
each clock is derived differently. This clock driver will probably work
correctly on EN751627, EN7528, and EN7580.

Signed-off-by: Caleb James DeLisle <cjd@cjdns.fr>
Reviewed-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>