MAINTAINERS: Update section for cpuidle-psci

Add myself as a co-maintainer for the cpuidle-psci driver and the
corresponding git-tree, which I am already using for this.

Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20250217140145.117086-1-ulf.hansson@linaro.org