sunxi: Move await_completion dram helper to dram.h

sunxi: Move await_completion dram helper to dram.h

The await_completion helper is already copy pasted between the sun4i and sun6i
dram code, and we need it for sun8i too, so lets make it an inline helper in
dram.h, rather then adding yet another copy.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>