ARM: OMAP2+: WDT: move init; add read_reset_sources pdata function pointer

The OMAP watchdog timer driver directly calls a function exported by
code in arch/arm/mach-omap2.  This is not good; it tightly couples
this driver to the mach-omap2 integration code.  Instead, add a
temporary platform_data function pointer to abstract this function
call.  A subsequent patch will convert the watchdog driver to use this
function pointer.

This patch also moves the device creation code out of
arch/arm/mach-omap2/devices.c and into arch/arm/mach-omap2/wd_timer.c.
This is another step towards the removal of
arch/arm/mach-omap2/devices.c.

Cc: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
[paul@pwsan.com: skip wd_timer device creation when DT blob is present]
Signed-off-by: Paul Walmsley <paul@pwsan.com>