dmaengine: mmp_tdma: fix the 'pointer from integer' warnings

the driver is using unsigned long type for storing the channel register base
"reg_base", this leads to bunch of warns when we try to use this as pointer. So
better use an iomem pointer type for this variable

drivers/dma/mmp_tdma.c: In function 'mmp_tdma_chan_set_desc':
drivers/dma/mmp_tdma.c:143: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:144: warning: passing argument 1 of '__raw_readl' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:144: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c: In function 'mmp_tdma_enable_chan':
drivers/dma/mmp_tdma.c:151: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:153: warning: passing argument 1 of '__raw_readl' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:153: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c: In function 'mmp_tdma_disable_chan':
drivers/dma/mmp_tdma.c:160: warning: passing argument 1 of '__raw_readl' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:160: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:164: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c: In function 'mmp_tdma_resume_chan':
drivers/dma/mmp_tdma.c:171: warning: passing argument 1 of '__raw_readl' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:171: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c: In function 'mmp_tdma_pause_chan':
drivers/dma/mmp_tdma.c:178: warning: passing argument 1 of '__raw_readl' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:178: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c: In function 'mmp_tdma_config_chan':
drivers/dma/mmp_tdma.c:263: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c: In function 'mmp_tdma_clear_chan_irq':
drivers/dma/mmp_tdma.c:269: warning: passing argument 1 of '__raw_readl' makes pointer from integer without a cast
drivers/dma/mmp_tdma.c:274: warning: passing argument 2 of '__raw_writel' makes pointer from integer without a cast

Signed-off-by: Vinod Koul <vinod.koul@intel.com>