can: flexcan: struct flexcan_regs: document registers not affected by soft reset

This patch documents which registers are not affected by a soft reset of the
flexcan IP core.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200922144429.2613631-5-mkl@pengutronix.de