i2c: rtl9300: introduce clk struct for upcoming rtl9607 support

i2c: rtl9300: introduce clk struct for upcoming rtl9607 support

In RTL9607C i2c controller, there is 10 bit CLK_DIV field for
setting the clock of i2c interface which depends on the rate
of i2c clk (which seems be fixed to 62.5MHz according to Realtek SDK).

Introduce the clk struct and the respective F_CLK_DIV and clk_div
which are going to be used in the upcoming patch for rtl9607c i2c
controller support addition.

devm_clk_get_optional_enabled() function was used for cleaner code
as it automatically returns NULL if the clk is not present, which is
going to be the case for RTL9300 and RTL9310 i2c controllers.

Signed-off-by: Rustam Adilov <adilov@disroot.org>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260401180648.337834-7-adilov@disroot.org