OMAP: DSS2: Add FEAT_DSI_REVERSE_TXCLKESC

OMAP3430 has RESETDONETXCLKESCx bits in the order following bitnumber
order for lanes 0, 1, 2: 28, 27, 26. OMAP3630 and later have them in
saner order: 24, 25, 26 (and 27, 28 for OMAP4).

This patch adds a dss_feature that can be used to differentiate between
those two orders of RESETDONETXCLKESCx bits.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>