drivers: clock: Add dumb clock driver for SiWx91x

This driver is mostly the initial seed for further implementation of a
real clock driver.

It doesn't allow the user to choose the clock source for the various
peripherals. The driver hardcodes some sane values.

Note that for now, the driver snps,designware-i2c does not support
"clocks" attribute. So this patch hardcode the clock configuration in
the init of the clock driver.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>