HID: Add Tablet Pick -> BTN_STYLUS2 mapping

Many tablet input drivers use BTN_STYLUS2 code to report a second
tablet stylus button. Yet, the generic HID input driver doesn't map it.
Apparently, because there is no corresponding usage in the HID Usage Tables.

This patch selects a compromise usage - "Tablet Pick" (0x46) to be mapped to
BTN_STYLUS2. Some tablets use "Eraser" usage, but mapping it to a button
wouldn't be entirely correct, because the specification says it may be "a
pressure quantity".

Signed-off-by: Nikolai Kondrashov <spbnick@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>