remtoteproc: maintain max notifyid

Some of the rproc drivers (STE modem specifically) needs to know
the range of the notification IDs used for notifying the device.

Maintain a variable in struct rproc holding the largest allocated
notification id, so low-level rproc drivers could access it.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
[ohad: rebase, slightly edit commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>