itdb9325fb: gamma profiles & register poking

I added some additional functionality:

#### Selectable Gamma profiles

    $ sudo modprobe itdb9325fb gamma=X

where X=0..6 to select a different gamma profile (to support LCD's with different characteristics).

#### Register poking

For fine-grained control over the registers, probably limited applicability (and possibly dangerous) other than adjusting gamma levels on-the-fly. 

    $ sudo -s
    # echo "0x0036=0x000F" > /sys/bus/platform/drivers/itdb28fb/itdb28fb.0/register
    # dmesg | tail -1
    [ 2159.928859] itdb28fb: writing 0x000F into register 0x0036.
    
#### Legacy header

I've been using 3.8.x kernels, and in order to get these compiling without breaking backwards compatibility, I re-added the HOTPLUG macros back in, in `legacy.h`.