arm64: psci: factor invocation code to drivers

To enable sharing with arm, move the core PSCI framework code to
drivers/firmware. This results in a minor gain in lines of code, but
this will quickly be amortised by the removal of code currently
duplicated in arch/arm.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Hanjun Guo <hanjun.guo@linaro.org>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
index 99c69a3..d8de6a8 100644
--- a/drivers/firmware/Kconfig
+++ b/drivers/firmware/Kconfig
@@ -5,6 +5,9 @@
 
 menu "Firmware Drivers"
 
+config ARM_PSCI_FW
+	bool
+
 config EDD
 	tristate "BIOS Enhanced Disk Drive calls determine boot disk"
 	depends on X86