dmaengine: stm32_mdma: add CONFIG_OF dependency

Without CONFIG_OF we get a build warning:

warning: (STM32_MDMA) selects DMA_OF which has unmet direct dependencies (DMADEVICES && OF)

This adds a dependency on CONFIG_OF. Since this means
we no longer need to select 'DMA_OF', I'm dropping that line
as well.

Fixes: a4ffb13c ("dmaengine: Add STM32 MDMA driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>