MIPS: add readl/write_be accessors

MIPS currently lacks the readl_be and writel_be accessors
which are required by BCM63xx for OHCI and EHCI support.
Let's define them globally for MIPS. This also fixes the
compilation of the bcm63xx defconfig against USB.

Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@linux-mips.org
Cc: Maxime Bizon <mbizon@freebox.fr>
Patchwork: http://patchwork.linux-mips.org/patch/793/


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