extcon: palmas: rename device tree binding matching with file name

The driver name is extcon/extcon-palmas.c and hence renaming the
device tree binding document to extcon-palmas.txt.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>