arm64: psci: remove ACPI coupling

The 32-bit ARM port doesn't have ACPI headers, and conditionally
including them is going to look horrendous. In preparation for sharing
the PSCI invocation code with 32-bit, move the acpi_psci_* function
declarations and definitions such that the PSCI client code need not
include ACPI headers.

While it would seem like we could simply hide the ACPI includes in
psci.h, the ACPI headers have hilarious circular dependencies which make
this infeasible without reorganising most of ACPICA. So rather than
doing that, move the acpi_psci_* prototypes into psci.h.

The psci_acpi_init function is made dependent on CONFIG_ACPI (with a
stub implementation in asm/psci.h) such that it need not be built for
32-bit ARM or kernels without ACPI support. The currently missing __init
annotations are added to the prototypes in the header.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Reviewed-by: Al Stone <al.stone@linaro.org>
Reviewed-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Will Deacon <will.deacon@arm.com>