Add support for probing the main system clock using the common clock
framework and its OF bindings.

Maintain ACPI compatibility by falling back to parse 'clock-frequency'.
Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Tested-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>