net: sh_eth: Add support for operation without clock framework

Add ifdeffery to allow operation without the clock framework
enabled. This is required on RZ/A1, as it does not have clock
driver yet.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Chris Brandt <chris.brandt@renesas.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>