MAINTAINERS: Add entry for SPMI subsystem

I have the hardware and I've been reviewing SPMI patches when
they come on the list. Add myself as a reviewer in this area and
add the linux-arm-msm list because people subscribed there also
have the hardware.

Cc: Kiran Gunda <kgunda@codeaurora.org>
Cc: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>