ARM: PSCI: Rework the DT handler slightly

ARM: PSCI: Rework the DT handler slightly

The way the PSCI DT update happens currently means we pull in
<asm/armv7.h> everywhere, including on ARMv8 and that in turn brings in
<asm/io.h> for some non-PSCI related things that header needs to deal
with.

To fix this, we rework the hook slightly.  A good portion of
arch/arm/cpu/armv7/virt-dt.c is common looking and I hope that when PSCI
is needed on ARMv8 we can re-use this by and large.  So rename the
current hook to psci_update_dt(), move the prototype to <asm/psci.h> and
add an #ifdef that will make re-use later easier.

Reported-by: York Sun <yorksun@freescale.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: York Sun <yorksun@freescale.com>
Cc: Ian Campbell <ijc@hellion.org.uk>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: York Sun <yorksun@freescale.com>