OMAP: DMA: Set thread reservation for high-priority channels

Reserve one thread for write and one for read for high-priority
channels. This configuration helps latency critical synchronized
transfers: memory-peripheral and/or peripheral-memory.

Change-Id: Ib8ecf32db126937820b1da04b183568c44143d64
Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: John Grossman <johngro@google.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>