HID: sony: Add touchpad support for NSG-MR5U and NSG-MR7U remotes

Sony's NSG-MR5U and NSG-MR7U remote controls have a full keyboard and a
touchpad.  The keyboard is already supported by the existing Linux
kernel and drivers but the touchpad is not recognized.  This patch adds
the coded needed to bring full functionality to the touchpad.

Note that these remotes use the vendor code for SMK even though they are
Sony branded.

Known limitations
- The built-in accelerometers are not supported by these changes
- When the Drag (Fn) key is used as a mouse button, the button is
automatically released when the key begins repeating.  There are two
  workarounds for this 1) Use the button behind the touchpad instead of
  the Drag (Fn) key or 2) Disable the key repeat functionality or
  increase the key repeat delay.

Signed-off-by: Todd Kelner <tsopdump@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>