Added driver for HiFiBerry Amp amplifier add-on board

The driver contains a low-level hardware driver for the TAS5713 and the
drivers for the Raspberry Pi I2S subsystem.

TAS5713: return error if initialisation fails

Existing TAS5713 driver logs errors during initialisation, but does not return
an error code. Therefore even if initialisation fails, the driver will still be
loaded, but won't work. This patch fixes this. I2C communication error will now
reported correctly by a non-zero return code.

HiFiBerry Amp: fix device-tree problems

Some code to load the driver based on device-tree-overlays was missing. This is added by this patch.