ARM: atmel: sama5: add sfr register header file

The SFR (special function registers) can be shared bwteen
sama5d3 and sama5d4 soc.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
[whitespace adoptions for 80 char compliance]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>