ARM: OMAP2+: hwmod: add parent_hwmod support

Add parent_hwmod pointer to omap_hwmod. This can be set to point to a
"parent" hwmod that needs to be enabled for the "child" hwmod to work.

This is used at hwmod setup time: when doing the initial setup and
reset, first enable the parent hwmod, and after setup and reset is done,
restore the parent hwmod to postsetup_state.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit.taneja@gmail.com>
[paul@pwsan.com: add kerneldoc documentation for parent_hwmod; note that it
 is a temporary workaround]
Signed-off-by: Paul Walmsley <paul@pwsan.com>