riscv: dts: thead: Introduce power domain nodes with aon firmware

riscv: dts: thead: Introduce power domain nodes with aon firmware

The DRM Imagination GPU requires a power-domain driver. In the T-HEAD
TH1520 SoC implements power management capabilities through the E902
core, which can be communicated with through the mailbox, using firmware
protocol.

Add AON node, which servers as a power-domain controller.

Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Reviewed-by: Drew Fustini <drew@pdp7.com>
Signed-off-by: Drew Fustini <drew@pdp7.com>