at91: introduce commom AT91_BASE_SYS

On all at91 except rm9200 and x40 have the System Controller starts
at address 0xffffc000 and has a size of 16KiB.

On rm9200 it's start at 0xfffe4000 of 111KiB with non reserved data starting
at 0xfffff000

This patch removes the individual definitions of AT91_BASE_SYS and
replaces them with a common version at base 0xfffffc000 and size 16KiB
and map the same memory space

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