ARM: at91: remove init_machine() as default is suitable

Since 883a106b0866ca8d75b5520bdb3ca1cf8e3730ba (ARM: default
machine descriptor for multiplatform) we can remove the SoC-specific
callback init_machine() to use the default code.
This cleans up the code and reduces the number of lines.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>