bcm27xx: add AMP2 to HifiBerry DAC+ / DAC+ Pro package

bcm27xx: add AMP2 to HifiBerry DAC+ / DAC+ Pro package

According to the vendor [1] these HATs share the same DT overlay:
hifiberry-dacplus. The PCM512x-compatible control unit is attached to
I2C, so the additional snd-soc-pcm512x-i2c kernel module is required.
Also explicitly note the Amp2 support to reduce confusion for those
users.

[1] <https://www.hifiberry.com/docs/software/configuring-linux-3-18-x/>
Signed-off-by: Torsten Duwe <[email protected]>
(added bcm27xx tag, changed commit message)
Signed-off-by: Christian Lamparter <[email protected]>