MAINTAINERS: Add Andrew as M: to ARM/NUVOTON NPCM ARCHITECTURE

Nuvoton's NPCM SoCs are part of their iBMC product line[1]. NPCM arch
patches have historically gone through Joel's tree along with ASPEED
changes due to their relevance to OpenBMC. Commit df5e674c
("MAINTAINERS: Switch ASPEED tree to shared BMC repository") does what
it says on the tin - we now have bmc/linux.git on git.kernel.org, and
I've picked up the maintainer role for it.

Document that I'm continuing to apply NPCM arch patches from the
openbmc@ list to the BMC tree for PRs to the SoC tree.

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Avi Fishman <avifishman70@gmail.com>
Cc: Drew Fustini <fustini@kernel.org>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Tali Perry <tali.perry1@gmail.com>
Cc: Tomer Maimon <tmaimon77@gmail.com>
Link: https://www.nuvoton.com/products/cloud-computing/ibmc/

 [1]
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>