kexec: add variant helper functions for handling memory regions

mem_regions_alloc_and_add() and mem_regions_alloc_and_exclude() are
functionally equivalent to, respectively, mem_regions_add() and
mem_regions_exclude() except the formers will re-allocate memory
dynamically when no more entries are available in 'ranges' array.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Tested-by: Bhupesh Sharma <bhsharma@redhat.com>
Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Simon Horman <horms@verge.net.au>