ARM: omap2: add select of TI_PRIV_EDMA

"ARM: OMAP: build mach-omap code only if needed" moved around the
ARCH_OMAP2PLUS stanza, but accidentally dropped the seleciton of
TI_PRIV_EDMA in the process. Add it back.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Olof Johansson <olof@lixom.net>