clk: rmobile: Move preboot clock shutdown to the driver

clk: rmobile: Move preboot clock shutdown to the driver

The MSTP registers were poked in boards/renesas/rcar-common/common.c
in arch_preboot_os hook thus far to shut down the clock before Linux
takes over. With DM, this is no longer needed and we can do the same
in the clock driver .remove callback. This patch adds such a .remove
callback for R8A7795 and R8A7796.

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