xen/arm: Warn user on cpu errata 832075

xen/arm: Warn user on cpu errata 832075

When a Cortex A57 processor is affected by CPU errata 832075, a guest
not implementing the workaround for it could deadlock the system.
Add a warning during boot informing the user that only trusted guests
should be executed on the system.
An equivalent warning is already given to the user by KVM on cores
affected by this errata.

Also taint the hypervisor as unsecure when this errata applies and
mention Cortex A57 r0p0 - r1p2 as not security supported in SUPPORT.md

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
[fix SUPPORT.md style, 3 printk lines instead of 4]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>