MIPS: ath79: Allow using ath79_ddr_wb_flush() from drivers

Move the declaration of ath79_ddr_wb_flush() to asm/mach-ath79/ath79.h
to allow using it from drivers. This is needed to move the CPU IRQ
driver to drivers/irqchip.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Alexander Couzens <lynxis@fe80.eu>
Cc: Joel Porquet <joel@porquet.org>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11502/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>