We use the U_BOOT_ prefix (i.e. U_BOOT_DRIVER) to declare a driver but
in every other case we just use DM_. Update the alias macros to use the
DM_ prefix.

We could perhaps rename U_BOOT_DRIVER() to DM_DRIVER(), but this macro
is widely used and there is at least some benefit to indicating it us a
U-Boot driver, particularly for code ported from Linux. So for now, let's
keep that name.

Signed-off-by: Simon Glass <sjg@chromium.org>