OMAP: hwmod: add API for slave idlemode setting

Some HW blocks have errata which requires specific slave idle mode
under certain conditions.

This patch adds an hwmod API to allow setting slave idlemode
ensuring that any SYSCONFIG register updates go through hwmod.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>