platform/x86: intel-vbtn: Split keymap into buttons and switches parts

Split the sparse keymap into 2 separate keymaps, a buttons and a switches
keymap and combine the 2 to a single map again in intel_vbtn_input_setup().

This is a preparation patch for not telling userspace that we have switches
when we do not have them (and for doing the same for the buttons).

Fixes: de9647ef

 ("platform/x86: intel-vbtn: Only activate tablet mode switch on 2-in-1's")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>