can: mcp25xxfd: Add Microchip mcp25xxfd CAN FD driver basics

This patch adds basic driver support for the Microchip mcp2517fd
CAN-FD controller.

The mcp2517fd is capable of transmitting and receiving standard data
frames, extended data frames, remote frames and Can-FD frames.
The mcp2517fd interfaces with the host over SPI.

This patch iprovides basic driver functionality:
setting up clocks and the infrastructure for the can and gpio portions
of the driver.

Datasheet:
* http://ww1.microchip.com/downloads/en/DeviceDoc/20005688A.pdf
Reference manual:
* http://ww1.microchip.com/downloads/en/DeviceDoc/20005678A.pdf
Errata:
* http://ww1.microchip.com/downloads/en/DeviceDoc/MCP2517FD-Silicon-Errata-and-Data-Sheet-Clarification-DS80000792A.pdf



--
Changelog:
  V4 -> V5: reorganisation of the patchset into smaller patches
            review of the whole driver code for better modularization
  V5 -> V6: Major refactoring as per feedback from Wilhelm Grandegger
            Fixing bugs reported by several other parties
            Split out of optimizations into separate patches
  V6 -> V7: added include linux/irqreturn.h (feedback by Eric Scholz)
            move can transmission into a separate patch to reduce
              the size of the individual patches to make the linux-can
              mailing-list happy...
  V7 -> V8: add support for mcp2518fd
            fixed mcp25xxfd_cmd_read_mask bug clearing bits
            allow setting SPI clock speeds via debugfs
            Incorprated Feedback by Marc Kleine-Budde
              sparse fixes
              makefile indenting

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>