USB: musb: make HAVE_CLK support optional

The Blackfin port doesn't support HAVE_CLK and the musb driver works fine
with support stubbed out, so take the existing Blackfin clk stubs and move
them to common musb code so we can drop the Kconfig dependency.

Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>