ARM: psci: add devicetree binding for describing PSCI firmware

This patch adds a new devicetree binding for describing PSCI firmware
to Linux.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>