AT91: Replace (undefined) AT91_ID_US* by the board specific values.

AT91: Replace (undefined) AT91_ID_US* by the board specific values.

AT91_ID_US0 / AT91_ID_US1 / AT91_ID_US2 were used but never defined.
Since they are never used outside the board specific files, they can
be replaced by the board specific AT91xxx_ID_US0 / AT91xxx_ID_US1 /
AT91xxx_ID_US2.

Bug spotted by Jesus Alvarez <jalvarez@micromint.com>.

Signed-off-by: Stelian Pop <stelian@popies.net>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>