ARM: rmobile: Stop using rcar-common/common.c on Gen3

Since the Gen3 clock driver now has a .remove callback, it is no
longer necessary to shut the clock down before booting Linux in the
arch_preboot_os hook. Stop using it and while doing so, remove all
the ad-hoc config options which this hook used.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>