dts: bindings: can: mcan: switch to using bosch,mram-cfg property

Switch the Bosch M_CAN devicetree binding to use a bosch,mram-cfg property
for specifying the memory layout of the Bosch M_CAN Message RAM. This is
identical to the Linux kernel devicetree binding for Bosch M_CAN IP core
based CAN controllers.

This introduces an offset cell which can be used for controllers with
shared Message RAM between Bosch M_CAN instances.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>