ARM: renesas: Drop unused pfc-r8a7790.h

This file is no longer used, the PFC pin controller driver is
superseded by drivers/pinctrl/renesas/pfc-r8a7790.c which
includes all the details of the hardware. Remove this file.
No functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>