Add code to read USB D+/D- and accessory ADCs, it is shown in HW
debug menu, might be useful in future for RB and/or the bootloader
to identify external USB chargers.
Change-Id: Ia48ca5e06bb7ddc52bb55abedde6734653ce8dba
PMU interrupts are used to detect USB Vbus, wall adaptor, accessories
and holdswitch. A thread is needed to poll the PMU throught I2C, ATM
it does nothing but showing the state of the inputs on the HW debug
menu, funcionallity for each individual input will be added in next
patches.
Change-Id: If93bf2044d1052729237a7fd1431c8493e09f1c7