[ARM] versatile: convert to clkdev and lookup clocks by device name

People often point to the Integrator/Versatile/Realview
implementations to justify using the consumer name as the sole
selector for clocks.

Eliminate this excuse by changing the Versatile implementation, so
it provides a better example of how it should be done.

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