Add KEYCODE_CAPTIONS mapping to LKC KEY_SUBTITLE

From https://github.com/torvalds/linux/blob/master/include/uapi/linux/input-event-codes.h,
KEY_SUBTITLE is near the TV codes. Note that 0x172 == 370.

  #define KEY_CHANNEL		0x16b
  #define KEY_FAVORITES		0x16c
  #define KEY_EPG			0x16d
  #define KEY_PVR			0x16e	/* Media Select Home */
  #define KEY_MHP			0x16f
  #define KEY_LANGUAGE		0x170
  #define KEY_TITLE		0x171
  #define KEY_SUBTITLE		0x172
  #define KEY_ANGLE		0x173
  #define KEY_FULL_SCREEN		0x174	/* AC View Toggle */
  #define KEY_ZOOM		KEY_FULL_SCREEN

You can also see on https://www.kernel.org/doc/html/v4.9/media/uapi/rc/rc-tables.html

 it is listed in the "Movie play control" section:

  KEY_SUBTITLE	Allow changing the subtitle

Change-Id: I7690269cddd00b787a939f87ba651db38f1160ac
Signed-off-by: Aman Gupta <aman@tmm1.net>