feat(allwinner): simplify CPU_SUSPEND power state encoding

Use the encoding recommended by the PSCI specification: four bits for
the power state at each power level.

SCPI provides no way to handshake an exit from a standby state, so the
only possible standby state is the architectural WFI state. Since WFI
can be used outside of PSCI, we do not allow passing in standby states.

Change-Id: I4b3b84e5c255ee58a25255a0cab5d7623425086e
Signed-off-by: Samuel Holland <samuel@sholland.org>