MAINTAINERS: Mark SPMI as maintained

MAINTAINERS: Mark SPMI as maintained

I can do more than just review patches here. The plan is to pick up
patches from the list and shuttle them up to gregkh. The korg tree will
be used to hold the pending patches. Move the list away from
linux-arm-msm to just be linux-kernel as SPMI isn't msm specific
anymore.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <linux-arm-msm@vger.kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20201207214204.1284946-1-sboyd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>