dma: tegra: make tegra_dma.chip_data a pointer to const data

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/dma/tegra20-apb-dma.c: In function 'tegra_dma_probe':
	drivers/dma/tegra20-apb-dma.c:1210:9: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

[ukl: split Laxman's patch to prevent warnings in the middle of the
series. Original patch was sent with msgid
1346217447-19730-1-git-send-email-ldewangan@nvidia.com]

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>