xen/arm: Relax ARM_SMCCC_ARCH_WORKAROUND_1 discovery

xen/arm: Relax ARM_SMCCC_ARCH_WORKAROUND_1 discovery

A recent update to the ARM SMCCC_ARCH_WORKAROUND_1 specification (see [1])
allows firmware to return a non zero, positive value, to describe that
although the mitigation is implemented at the higher exception level,
the CPU on which the call is made is not affected.

Relax the check on the return value from ARM_WORKAROUND_1 so that we
only error out if the returned value is negative.

[1] https://developer.arm.com/support/security-update/downloads
"Firmware interfaces for mitigating CVE-2017-5715 System Software on Arm
Systems"

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>