MAINTAINERS: adjust PISTACHIO SOC SUPPORT after its retirement

Commit 104f942b

 ("MIPS: Retire MACH_PISTACHIO") removes
./arch/mips/pistachio/ and ./arch/mips/configs/pistachio_defconfig, but
misses to adjust the corresponding section PISTACHIO SOC SUPPORT
in MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches    F:    arch/mips/configs/pistachio*_defconfig
  warning: no file matches    F:    arch/mips/pistachio/

As James Hartley is not reachable with the provided email address, the
remaining dtsi file, arch/mips/boot/dts/img/pistachio.dtsi, must be
maintained by its only user pistachio_marduk.dts, which is part of MARDUK
(CREATOR CI40) DEVICE TREE SUPPORT.

Add maintenance of pistachio.dtsi to that section and drop the PISTACHIO
SOC SUPPORT after its retirement.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Acked-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>