ARM: at91: move SoC detection to its own driver

To simplify machine init and as the soc_device struct is not used as the
parent for on-chip devices anymore, move SoC detection to its own driver.

Change in dmesg:
 - before:
DMA: preallocated 256 KiB pool for atomic coherent allocations
AT91: Detected SoC family: sama5d2
AT91: Detected SoC: sama5d27, revision 0
No ATAGs?
clocksource: tcb_clksrc: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 184217874325 ns
at_xdmac f0010000.dma-controller: 16 channels, mapped at 0xe085b000
SCSI subsystem initialized

 - after:
DMA: preallocated 256 KiB pool for atomic coherent allocations
No ATAGs?
clocksource: tcb_clksrc: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 184217874325 ns
at_xdmac f0010000.dma-controller: 16 channels, mapped at 0xe0859000
AT91: Detected SoC family: sama5d2
AT91: Detected SoC: sama5d27, revision 0
SCSI subsystem initialized

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>