clk: Add missing clk_get_sys() stub

When compiling with the COMPILE_TEST option set, the clps711x does not
compile because of the clk_get_sys() noop stub missing.

Signed-off-by: Daniel Lezcano <[e-mail]>
Reviewed-by: Michael Turquette <[e-mail]>

include/linux/clk.h

···
 	return NULL;
 }
 
+static inline struct clk *clk_get_sys(const char *dev_id, const char *con_id)
+{
+	return NULL;
+}
 #endif
 
 /* clk_prepare_enable helps cases using clk_enable in non-atomic context. */