net: ravb: Add clock handling support

Add support for enabling and disabling the clock using the clock
framework based on the content of OF instead of doing it manually
in the board file.

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