xshm: Platform device for XSHM

xshm: Platform device for XSHM

This patch implements the configuration handling and interface to XSHM
drivers. Specifically:
- reception of gen-netlink requests from user-space.
- calculating address of where to put information in shared memory, i.e
  the offset of: IPC-TOC, channel-descriptors, read/write indexes for each
  directional channel, and data area of each channel.
- formatting of the shared memory area, so modem can read out configuration
  data.
- Creation of configuration data given to the XSHM drivers: xshm_chr and
  caif_xshm.
- Platform device management.
- Channel management such as open/close.
- Modem start-up synchronization events.
- C2C power request indications, requesting power-on when transmit is ongoing,
  and power-off upon inactivity timeout.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>