ARM: at91: remove at91_boot_soc

at91_boot_soc and at91_init_soc structures are not used by any SoC, remove
their use. Also remove all the now empty SoC files.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: different organization of the patches]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>