can: mcp25xxfd: add regmap infrastructure

This patch adds the regmap infrastructure for the Microchip MCP25xxFD SPI CAN
controller family. The actual driver is added in the next commit.

Tested-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200918172536.2074504-3-mkl@pengutronix.de