AT91: add header files, update cpu.c, add RTC support

This is a series of 3 patches:

[PATCH 1/3] AT91: add two new header files for RTT and GPBR
- adds at91_rtt.h and at91_gpbr.h
- condenses GPBR usage assignment into one file (at91_gpbr.h)
- required to compile the following two patches

[PATCH 2/3] AT91: update cpu.c to use GPBR definitions from at91_gpbr.h
- no functional change, just make use of at91_gpbr.h

[PATCH 3/3] AT91: add RTT and GPBR based RTC support
- adds kernel compatible RTC handling to u-boot using the RTT and one GPBRegister