MAINTAINERS: Add a git for the DT IDLE PM DOMAIN

Let' make it clear that we have git to manage the corresponding patches for
for the DT IDLE PM DOMAIN.

Cc: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20240404110018.568143-1-ulf.hansson@linaro.org