Input: mms114 - add support for touch keys

MELFAS MMS114 and similar touchscreens have support for touch keys.
Enable support of them in the driver. The keycodes to emit can be
controlled by the linux,keycodes DT property.

Sidenote - the MAX_TOUCHKEYS value is set to 15, as that's the
maximum value that the ID field can contain. I don't have access
to any datasheets that could confirm or deny whether this is accurate.

Most downstream drivers I've been able to find only use up to 2 keys
(though I did find a driver that mentioned up to 4, but only 2 were
used). They don't have any checks for a maximum keycode value, it is
just extracted from the ID bits (0xf mask).

The drivers I've been able to find also don't use touch ID 0; I assume
that it is never used, so the keycodes provided in the DT start from
touch ID 1. I suppose this is in-line with the regular behavior
for touch IDs in touchscreen events, as there the provided touch ID
is always lowered by 1, which would cause an overflow if it was 0...
Just in case, we quietly return if the touch ID is set to 0 here.

The implementation of the linux,keycodes property handling code was
adapted from the msg2638 driver.

Signed-off-by: Artur Weber <aweber.kernel@gmail.com>
Link: https://lore.kernel.org/r/20230714100424.29798-3-aweber.kernel@gmail.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>