soc: add aspeed folder and misc drivers

Create a SoC folder for the ASPEED parts and place the misc drivers
currently present into this folder.  These drivers are not generic part
drivers, but rather only apply to the ASPEED SoCs.

Signed-off-by: Patrick Venture <venture@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>