ata: ahci_brcmstb: remove unused definitions

Remove unused definitions, and this is to avoid confusion with MIPS-based
platforms.

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