drivers: clk: Add a managed API to get clocks from the device-tree

Add devm_clk_get(), devm_clk_get_optional() to get clocks from the
device-tree. The clocks is automatically released and the data structure
freed when the device is unbound.
Also add devm_clk_put() to release the clock and free the data structure
manually.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>