MIPS: ath79: Improve the DDR controller interface

The DDR controller need to be used by the IRQ controller to flush
the write buffer of some devices before running the IRQ handler.
It is also used by the PCI controller to setup the PCI memory windows.

The current interface used to access the DDR controller doesn't
provides any useful abstraction and simply rely on a shared global
pointer.

Replace this by a simple API to setup the PCI memory windows and use
the write buffer flush independently of the SoC type. That remove the
need for the shared global pointer, simplify the IRQ handler code.

[ralf@linux-mips.org: Folded in Alban Bedel's follup fix.]

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Qais Yousef <qais.yousef@imgtec.com>
Cc: Wolfram Sang <wsa@the-dreams.de>
Cc: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9773/
Patchwork: http://patchwork.linux-mips.org/patch/10543/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>