max17042: Align register definitions with data sheet and init appnote

align the register names with max17042 data sheet removing
registers that are marked reserved that are not used.

Add register definitions defined in the maxim initialization appnote

Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>