ARM: spectre-v2: warn about incorrect context switching functions

Warn at error level if the context switching function is not what we
are expecting.  This can happen with big.Little systems, which we
currently do not support.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Boot-tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>