clkdev: add clkdev_create() helper

Add a helper to allocate and add a clk_lookup structure.  This can not
only be used in several places in clkdev.c to simplify the code, but
more importantly, can be used by callers of the clkdev code to simplify
their clkdev creation and registration.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>