MAINTAINERS: update entry for TI's CPSW driver

Mugunthan V N, who was reviewing TI's CPSW driver patches is
not working for TI anymore and wont be reviewing patches for
that driver.

Drop Mugunthan as the maintiainer for this driver.

Grygorii continues to be a reviewer. Dave Miller applies the
patches directly and adding a maintainer is actually
misleading since get_maintainer.pl script stops suggesting
that Dave Miller be copied.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>