omap: spi: Drop CONFIG_OMAP3_SPI_D0_D1_SWAPPED support

This particular quirk is not enabled in any config files today.  It does
however exist and is handled correctly in device trees and via
CONFIG_DM_SPI.  So we drop the symbol now and add a comment to indicate
that any (new) boards that require this quirk need to enable DM_SPI
instead.

Signed-off-by: Tom Rini <trini@konsulko.com>