mtk: Add new atoms

Add atoms that have already been added to ril plugins: call barring,
radio settings and another GPRS context for MMS.
mtkmodem: Fix double registration to voicecall

mtk modem was registering twice to the voicecall atom: once in
rilmodem voicecall driver and once in mtkmodem voicecall driver.
mtk: Fix devinfo on boot

Serial (IMEI) and Revision modem properties were not shown when
re-starting the phone, as the devinfo atom was created when the radio
state was still unavailable.
Merge pull request #82 from alfonsosanchezbeato/mtk-update

Mtk update