[DRIVERS] Refurbish clocking interface for drivers

The clocking interface (i.e. arch_clk.h) is totally optional for
architectures (e.g. x86 will probably never use it) and there is
also generic device tree based clocking framework possible for
all architectures. Due to this facts, arch_clk.h is not right
place for defining clocking interface instead we rename this to
drivers/include/drv/clk.h.

In addition to above, this patch also does following changes:
1. Improve clock lookup APIs (clkdev) and make it linux like
2. Use clock lookup APIs for realview-eb-mpcore, realview-pb-a8,
   VExpress-A9 and VExpress-A15 hosts
3. Add openconf option and stubs for up-coming commong device
   tree based clocking framework

The above changes not only refurbishes our clocking interface but
also brings it more closer linux clocking interface hence enabling
us to provide linux-like linux/clk.h and linux/clkdev.h.

Signed-off-by: Anup Patel <anup@brainfault.org>