MAINTAINERS: Add pistachio SoC Support

MAINTAINERS: Add pistachio SoC Support

The Pistachio SoC from Imagination Technologies currently has no
entry in the MAINTAINERS file, so add one.

Signed-off-by: James Hartley <james.hartley@imgtec.com>
Reviewed-by: Ionela Voinescu <ionela.voinescu@imgtec.com>
Cc: davem@davemloft.net
Cc: geert@linux-m68k.org
Cc: gregkh@linuxfoundation.org
Cc: akpm@linux-foundation.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: ionela.voinescu@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/13755/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>