ARM: at91: PIT: (Almost) remove the global variables

The timer driver is using some global variables to define some variables it has
to use in most of its functions, like the base address.

Use some container_of calls to have a single dynamic (and local) variable to
hold this content.

The only exception is in the !DT case, where the call chain to
at91sam926x_ioremap_pit and then at91sam926x_pit_init as init_time makes it
hard for the moment to pass the physical address of the timer.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>