CLKDEV: provide helpers for common clock framework

The common clock framework allocates clocks dynamically.  Provide a
set of helpers to streamline the clkdev registration of the clock
lookups to avoid repetitive code sequences.

Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>