ARC: clk: introduce HSDK CGU clock driver

Synopsys HSDK clock controller generates and supplies clocks to various
controllers and peripherals within the SoC.

Each clock has assigned identifier and client device tree nodes can use
this identifier to specify the clock which they consume. All available
clocks are defined as preprocessor macros in the
dt-bindings/clock/snps,hsdk-cgu.h header and can be used in device
tree sources.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>