clk: mvebu: Remove inappropriate __init tagging

If the Orion WDT driver is built as a module, an opps occurs during
clk lookup when calling mvebu_clk_gating_get_src().  Remove the
inappropriate __init tag so the function is available for modules
after kernel init.

Signed-off-by: Joshua Coombs <josh.coombs@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>