xen/arm: vpsci: Removing dummy MIGRATE and MIGRATE_INFO_UP_CPU

xen/arm: vpsci: Removing dummy MIGRATE and MIGRATE_INFO_UP_CPU

The PSCI call MIGRATE and MIGRATE_INFO_UP_CPU are optional and
implemented as just returning PSCI_NOT_SUPPORTED (aka UNKNOWN_FUNCTION
for SMCCC).

The new SMCCC framework is able to deal with unimplemented function and
return the proper error code. So remove the implementations for both
function.

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