power: pm8921-charger: expose some properties to the userspace

Prepare for implementation of battery current calibration code
running in userspace. The userspace is expected to read these
parameters and program a battery current value.

The buck regulation loop can be read here
/sys/module/pm8921_charger/parameters/reg_loop

The maximum charge current can be read here
/sys/module/pm8921_charger/parameters/max_chg_ma

The present battery charge current can be read here
/sys/module/pm8921_charger/parameters/ibatmax_ma

Also increase the upper limit on the current value we can program
in the IBATMAX register. This is to accommodate larger batteries
and also give the calibration algorithm some room to increase the
battery current.

Change-Id: Ic68717f4410170814d3e3b141de77b35eca12f7b
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>