dt-bindings: HID: i2c-hid: elan: Introduce Elan ekth6a12nay

The Elan ekth6a12nay touch screen chip same as Elan eKTH6915 controller
has a reset gpio. The difference is that they have different
post_power_delay_ms.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Zhaoxiong Lv <lvzhaoxiong@huaqin.corp-partner.google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>