ARM: OMAP2+: hwmod: add support for blocking WFI when a device is active

Apparently, on some OMAPs, the MPU can't be allowed to enter WFI while
certain peripherals are active.  It's not clear why, and it's likely
that there is simply some other bug in the driver or integration code.
But since the likelihood that anyone will have the time to track these
problems down in the future seems quite small, we'll provide a
flag, HWMOD_BLOCK_WFI, to mark these issues in the hwmod data.

Signed-off-by: Paul Walmsley <paul@pwsan.com>