xen/arm: vpsci: Move PSCI function dispatching from vsmc.c to vpsci.c

xen/arm: vpsci: Move PSCI function dispatching from vsmc.c to vpsci.c

At the moment PSCI function dispatching is done in vsmc.c and the
function implementation in vpsci.c. Some bits of the implementation is
even done in vsmc.c (see PSCI_SYSTEM_RESET).

This means that it is difficult to follow the implementation and also
it requires to export functions for each PSCI function.

Therefore move PSCI dispatching in two new functions do_vpsci_0_1_call
and do_vpsci_0_2_call. The former will handle PSCI 0.1 calls while the
latter 0.2 or later calls.

At the same time, a new header vpsci.h was created to contain all
definitions for virtual PSCI and avoid confusion with the host PSCI.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>