xen/arm: smccc-1.1: Make return values unsigned long

xen/arm: smccc-1.1: Make return values unsigned long

An unfortunate consequence of having a strong typing for the input
values to the SMC call is that it also affects the type of the
return values, limiting r0 to 32 bits and r{1,2,3} to whatever
was passed as an input.

Let's turn everything into "unsigned long", which satisfies the
requirements of both architectures, and allows for the full
range of return values.

Reported-by: Stefano Stabellini <stefanos@xilinx.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>