remoteproc/omap: two Kconfig fixes

1. Depend on OMAP_IOMMU instead of selecting it, to fix an unmet
   direct dependency of it (and its imminent build error)
2. Set default to 'no' (achieved implicitly by dropping the 'default'
   line)

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Mark Grosen <mgrosen@ti.com>
Cc: Suman Anna <s-anna@ti.com>
Cc: Fernando Guzman Lugo <fernando.lugo@ti.com>
Cc: Rob Clark <rob@ti.com>
Cc: Ludovic BARRE <ludovic.barre@stericsson.com>
Cc: Loic PALLARDY <loic.pallardy@stericsson.com>
Cc: Omar Ramirez Luna <omar.luna@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>