ata: ahci_brcmstb: add support for MIPS-based platforms

The BCM7xxx ARM-based and MIPS-based platforms share a similar hardware
block for AHCI SATA3.

This new compatible string, "brcm,bcm7425-ahci", may be used for most
MIPS-based platforms of 40nm process technology.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>