ARM: at91: remove at91_dt_initialize and machine init_early()

Move the ramc initialization to pm.c as it is the only user left.
This allows us to get rid of at91_dt_initialize() that was the only one called
by the init_early() function pointer of struct machine_desc.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: adapt patch to newer series]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>