ARM: OMAP2+: Fix DMA hang after off-idle

ARM: OMAP2+: Fix DMA hang after off-idle

Commit e6489e1a2de2 (dmaengine: omap-dma: move IRQ handling to omap-dma)
added support for handling interrupts in the omap dmaengine driver
instead of the legacy driver. Because of different handling for
interrupts this however caused omap3 to hang eventually after hitting
off-idle.

Any of the virtual 32 DMA channels can be assigned to any of the
four DMA interrupts. So commit e6489e1a2de2 made the omap dmaengine
driver to use the second DMA interrupt while keeping the legacy code
still using the first DMA interrupt.

This means we need to save and restore both IRQENABLE_L1 in addition
to IRQENABLE_L0. As there is a chance that the DSP might be using
IRQENABLE_L2 or IRQENABLE_L3 lines, let's not touch those until
this has been confirmed. Let's just add a comment to the code for
now.

Fixes: e6489e1a2de2 (dmaengine: omap-dma: move IRQ handling to omap-dma)
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>