ofw_cpu: collapse some #ifdef code

Mainly, to avoid repeating the list of architectures, #define HAS_CLK.

Further, split the clk code into a helper function, which is a stub in
the !HAS_CLK case. This aids in overall legibility.

While here, add one separating whitespace, again for legibility.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48149