mips: bmips: enable RAC on BMIPS4350

The data RAC is left disabled by the bootloader in some SoCs, at least in
the core it boots from.
Enabling this feature increases the performance up to +30% depending on the
task.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
[ rework code and reduce code duplication ]
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>