ARM: at91: SAMA5D4 SoC detection code and low level routines

SoC identification code, kernel uncompress and low level
debugging routines update.
On SAMA5D4, DBGU is at another address AT91_BASE_DBGU2 so another
round of detection is needed. We also had to differentiate with
SAMA5D3 SoC family and rename some variables.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>