msm: mdm: add support for multiple external modems

The mdm driver previously assumed that it would only need to support
one external modem at a time. This is no longer true. The changes now
support a arbitrary number of modems. Each modem must have its own
platform data.

Change-Id: I43918f92935b4b447a46d02721239fed0b5e877e
Signed-off-by: Ameya Thakur <ameyat@codeaurora.org>