xen/arm: smccc: Implement SMCCC v1.1 inline primitive

xen/arm: smccc: Implement SMCCC v1.1 inline primitive

One of the major improvement of SMCCC v1.1 is that it only clobbers the
first 4 registers, both on 32 and 64bit. This means that it becomes very
easy to provide an inline version of the SMC call primitive, and avoid
performing a function call to stash the registers that woudl otherwise
be clobbered by SMCCC v1.0.

This patch has been adapted to Xen from Linux commit f2d3b2e8759a. The
changes mades are:
    - Using Xen coding style
    - Remove HVC as not used by Xen
    - Add arm_smccc_res structure

 Reviewed-by: Robin Murphy <robin.murphy@arm.com>
 Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
 Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
 Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>