ARM: OMAP2+: clockdomain: Reintroduce SW_SLEEP Support

Since commit 65aa94b2 (ARM: OMAP4: clockdomain/CM code: Update supported
transition modes), on OMAP4, all CLKDMs support HW_AUTO so this is used
instead of SW_SLEEP for the idling of clockdomains. However, additional
SoCs now leverage the OMAP4 clockdomain code so update it to use SW_SLEEP
if the clockdomain data specifies that the CLKDM has the
CLKDM_CAN_FORCE_SLEEP flag set rather than using HW_AUTO for both cases.

Without this patch, clockdomain handling is broken on AM43xx and no
clockdomains are actually being put into idle on this platform. Any
attempt to idle them results in the HW_AUTO value (0x3) being written
to them with no apparent effect.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
[paul@pwsan.com: added extra explanatory text from patch set intro]
Signed-off-by: Paul Walmsley <paul@pwsan.com>