platform: Add HSM implementation for Allwinner D1

Allwinner D1 contains a "PPU" power domain controller which can
automatically power down/up the CPU power domain. This power domain
includes the C906 core along with its CLINT and PLIC.

This HSM implementation supports non-retentive hart suspend by:
  1) Saving/restoring state that is lost during hart suspend,
  2) Performing cache maintenance before/after hart suspend,
  3) Configuring wakeup sources before hart suspend, and
  4) Asking the PPU to power down the hart when it enters WFI.

Since this HSM implementation is for a single-core SoC, it does not need
to worry about concurrency or saving multiple instances of state.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>