ARM: at91: change "Unknown" qualifier SoC subtype handling

An AT91 SoC that doesn't have a subtype is shown as "Unknown"
in the Linux log message which is not correct.
This was leading to confusion so, add a "none" qualifier to
the subtype property and set this one in the appropriate cases.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>