backlight: Add support for MacBook 5, MacBook Air 2, and MacBook Pro 5

This patch adds support for the new Apple models incorporating an Nvidia
chipset.  Apple still uses the same protocol as on older models, but the
registers moved to a different address. To do this, two sets of functions
are added for the Intel/Nvidia chipset models and passed by the DMI_MATCH
function.

The initial code has been contributed by Hu Gang <hugang@soulinfo.com>.

The driver is known to work on MacBook Pro 3, MacBook Pro 4 and MacBook
Pro 5.

Its known to work with limitations on MacBook 5 / MacBook Air 2. Changing
brightness within X doesn't work, if using Nvidia's proprietary graphics
driver with no known fix at present. Changing brightness on a text console
or using the open-source driver does work.

MacBook Pro 5 has a known bug where the initial brightness after bootup is
the last recently used brightness (in Mac OSX), while the firmware reports
maximum. Impossible to fix.

[akpm@linux-foundation.org: build fix]
[rpurdie@linux.intel.com: Rebased the patch against latest git]
Signed-off-by: Mario Schwalbe <schwalbe@inf.tu-dresden.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>