dmaengine: dw: fix typo in Kconfig

platfroms -> platforms

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: fed42c19

 ("dma: dw: add PCI part of the driver")
Cc: Viresh Kumar <vireshk@kernel.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>