MAINTAINERS, git-mailrc: cortina: add Custodian for Cortina Access Inc.

Assign Alex Nemirovsky as custodian for all
Cortina Access (CA) for ARM and MIPS based SoCs.

Currently Cortina Access CAxxxx family of SoCs support both
ARM and MIPS ISA.  Drivers have cross platform support for
both architectures.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Alex Nemirovsky <alex.nemirovsky@cortina-access.com>