xivo-yealink: add basic EXP20 support (unofficial)

The EXP20 is an expansion module for the T27P and T29G. Like the EXP40,
you can program up to 40 function keys on it (20 functions keys per page
and 2 pages per extension module), but contrary to the EXP40, it does
not have physical keys to switch from one page to the other: you instead
have to manually configure a function key to do the page switching.
Here's an example configuration:

   expansion_module.1.key.1.type = 37
   expansion_module.1.key.1.label = PAGE 1

   expansion_module.1.key.21.type = 37
   expansion_module.1.key.21.label = PAGE 2

See http://projects.xivo.io/issues/6100#note-7 for more details.

Patch contributed by Julien Revert.