MAINTAINERS: Add myself as a maintainer for MMIO m_can

Since I refactored the code to create a m_can framework and we
have a MMIO MCAN IP as well add myself to help maintain the code.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>