Add ethosu_config_select platform hook

Allow config and hence port selection to be done at run-time based on
region address, avoiding need to recompile C code for linker map
changes, and permitting an application to have one model in SRAM and
another in external memory.

Change-Id: I425ee1ad2598b65a3c891261a7eff00e9565bdbe
Signed-off-by: Kevin Bracey <kevin.bracey@alifsemi.com>
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>