clk: nomadik: fix multiplatform problem

The Nomadik debugfs screws up multiplatform boots if debugfs
is enabled on the multiplatform image, since it's a simple
initcall that is unconditionally executed and reads from certain
memory locations.

Fix this by checking that the driver has been properly
initialized, so a base offset to the Nomadik SRC controller
exists, before proceeding to register debugfs files.

Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Mike Turquette <mturquette@linaro.org>