ARM: OMAP2+: add user and fifo info to mailbox platform data

The different generations of OMAP2+ SoCs have almost the same
mailbox IP, but the IP has configurable parameters for number
of users (interrupts it can generate out towards processors)
and number of fifos (the base unidirectional h/w communication
channel). This data cannot be read from any registers, and so
has been added to the platform data.

This data together with the interrupt-type configuration can be
used in properly figuring out the number of registers to save
and restore in the OMAP mailbox driver code.

Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Suman Anna <s-anna@ti.com>