MIPS: PowerTV: Streamline access to platform device registers

Pre-compute addresses for the basic ASIC registers. This speeds up access
and allows memory for unused configurations to be freed. In addition,
uninitialized register addresses will be returned as NULL to catch bad
usage quickly.

Signed-off-by: David VomLehn <dvomlehn@cisco.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/806/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>