serial: sh-sci: Dynamic clock management depends on HAVE_CLK.

Presently this is conditionalized on sh, and disabled for sh64.
Now that SH-5 ties in to the clock framework, the sh64 exception
can be dropped. Additionally, ARM will want to use the same hooks
once SH-Mobile G3 grows clock framework support, so switch these
paths over to HAVE_CLK now.

Once the H8 and ARM sh-sci users hook up HAVE_CLK, the driver can
be switched over to having an outright dependency on it and the
ifdefs can go away.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>