pdbg: Bump version to v2.2

Upstream changes:

 - libpdbg API clean-up
 - Path based targetting
 - POWER8 sreset fixes
 - gdbserver

This also adds the v back to the version string that pdbg's build system
uses, following the recent buildroot changes to remove "v" from package
versions.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>