gtimer: improve bind framework handler

The bind framework handler does not check the API id requested before
returning a valid implementation of a API. This is not scalable as well
while adding additional API support. So improve the implementation of
the bind API by checking API id requested before returning a valid API
reference.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: Icb9e55b25ecd3f9415750aed239f6997e01b0316