soc: qti_battery_debug: Add votables R/W support

Add debugfs nodes to read and set votables from the firmware running on
the remote subsystem (e.g. DSP).

Also modify error handling logic in battery_dbg_add_debugfs().

Change-Id: I50f191add22cd500e04475f264e86cf16c6be48f
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>