MAINTAINERS: Add maintainers for MIPS core drivers

Add Thomas and myself as maintainers of the MIPS CPU and GIC IRQchip, MIPS
GIC timer and MIPS CPS CPUidle drivers.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>