Blackfin: dpmc: use module_platform_driver macro

This patch removes some code duplication by using
module_platform_driver.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>