ARCv2: IOC: use @ioc_enable not @ioc_exist where intended

if user disables IOC from debugger at startup (by clearing @ioc_enable),
@ioc_exists is cleared too. This means boot prints don't capture the
fact that IOC was present but disabled which could be misleading.

So invert how we use @ioc_enable and @ioc_exists and make it more
canonical. @ioc_exists represent whether hardware is present or not and
stays same whether enabled or not. @ioc_enable is still user driven,
but will be auto-disabled if IOC hardware is not present, i.e. if
@ioc_exist=0. This is opposite to what we were doing before, but much
clearer.

This means @ioc_enable is now the "exported" toggle in rest of code such
as dma mapping API.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>