MAINTAINERS: add entries for misc. RISC-V SoC drivers and devicetrees

MAINTAINERS: add entries for misc. RISC-V SoC drivers and devicetrees

Following some discussion both on & off list, I have volunteered to take
over maintaining the miscellaneous RISC-V devicetrees & soc drivers from
Palmer to ease his load.

So far only SiFive and Microchip have stuff in drivers/soc. For the
former, a SiFive entry exists with a dead GitHub repo - so remove that
to avoid confusion since the patches for drivers/soc & devicetrees will
be routed via my tree & other drivers go through their subsystem trees.
The Microchip directory only contains a RISC-V driver for now, but is
likely to contain drivers for other archs in the future. To that end,
change the PolarFire SoC entry to specifically mention the RISC-V driver
& the new directory level entry does not mention an architecture.

CC: Arnd Bergmann <arnd@arndb.de>
CC: Nicolas Ferre <nicolas.ferre@microchip.com>
CC: Palmer Dabbelt <palmer@dabbelt.com>
Link: https://lore.kernel.org/linux-riscv/mhng-e4210f56-fcc3-4db8-abdb-d43b3ebe695d@palmer-ri-x1c9a/
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20221109212219.1598355-2-conor@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>