Add PLLV3_SYS support by adding set/get_rate() for PLLV3_SYS but keeping
generic enable()/disable(). Add a different driver because ops are
different respect to GENERIC/USB.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>