Align OF_getencprop_alloc API with OF_getencprop and OF_getprop_alloc

Change OF_getencprop_alloc semantics to be combination of malloc and
OF_getencprop and return size of the property, not number of elements
allocated.

For the use cases where number of elements is preferred introduce
OF_getencprop_alloc_multi helper function that copies semantics
of OF_getencprop_alloc prior to this change.

This is to make OF_getencprop_alloc and OF_getencprop_alloc_multi
function signatures consistent with OF_getencprop_alloc and
OF_getencprop_alloc_multi.

Functionality-wise this patch is mostly rename of OF_getencprop_alloc
to OF_getencprop_alloc_multi except two calls in ofw_bus_setup_iinfo
where 1 was used as a block size.