add z1/z2 read support to DSi codec/touch

[asiekierka]

This should allow pressure sensitivity support on DSi/3DS consoles.

Testing with the touch_area example on my New 3DS:

• pen touches seem to return a "Touch Area" value in the 0300 (lighter) - 0200 (heavier) range,
• finger touches - in the 0300 (lighter) - 0080 (heavier) range.

Based on what I've read, this seems to be correct behaviour for functioning pressure detection.

Further testing is most likely required (particularly on an actual DSi console, as I do not currently own one - Arisotura has verified on her DSi and it seems to be working, at least).

[Peter0x44]

I'd be happy to test this on my DSi - is anything specific needed?