HID: asus: add keycodes for 0x6a, 0x4b, and 0xc7

These two keys are found on some models with dual display.
- 0x6a is intended for controlling the secondary screen brightness.
- 0x4b is intended for toggling the arrow keys between arrows and
  page up / page down.
This key is found on a slightly modified keyboard layout.
- 0xc7 is intended to cycle through keybvoard brightnesses (upwards) but
  there is not suitable existing code for this behaviour. Using
  `KEY_KBDILLUMTOGGLE` is different behaviour to Windows but at least
  is picked up by many desktops already.

Signed-off-by: Luke D. Jones <luke@ljones.dev>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>