at91: use structure to store the current soc

instead of reading the registers everytime

the current implementation respect the following constrain:
 - allow 1 to n soc to be enabled
 - allow to have a virtual cpu type and subtype
 - always detect the cpu type and subtype and report it
 - detect if the soc support is enabled
 - prepare for sysfs export support
 - drop soc specific code via compiler when the soc not enabled
   (via cpu_is_xxx)

Today if we read the exid we will have the same value for 9g35 and 9m11
and we will need to check the cidr too

with the new implementation we just need to check the soc subtype

this will also allow to have specific virtual subtype for rm9200 which the
board will have to specify via at91rm9200_set_type(int) as we have no way to
detect it.

this implementation is inspired by the SH cpu detection support

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Patrice Vilchez <patrice.vilchez@atmel.com>