xtensa: xtfpga: use clock provider, don't update DT

Instead of querying hardcoded FPGA frequency register and then updating
clock-frequency property in specificly named DT nodes in machine setup
code register a clock provider that returns fixed-rate clock, configured
by register specified in DT. This way we have less magic/hardcoded names
and use more existing common clock framework code.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>