remoteproc: Add STE modem driver

Add support for the STE modem shared memory driver.
This driver hooks into the remoteproc framework
in order to manage configuration and the virtio
devices.

This driver adds custom firmware handlers, because
STE modem uses a custom firmware layout.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
cc: Linus Walleij <linus.walleij@linaro.org>
cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
[ohad: validate mdev->ops, move setup() to probe/remove, trivial style changes]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>