MAINTAINERS: Add entry for Intel PMC mux driver

I will be maintaining the Intel PMC mux control driver.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20200507150900.12102-5-heikki.krogerus@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>