ARM: davinci: PM: rework init, remove platform device

Remove fake platform device used for PM init.  Move pdata values which
are common across all current platforms into pm.c.

Also, since PM is only used on da8xx, remove davinci_pm_init() from
common init code, and only use in da850/omapl138 board files that are
currently creating the fake platform_device.

Suggested-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
[nsekhar@ti.com: subject line adjustment]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>