mmc: dw_mmc: convert copy of struct device in struct dw_mci to a reference

The 'struct dw_mci' maintains a copy of the pdev->dev instance instead of
maintaining a reference to that 'struct device' instance. Any resource
allocated using the device resource management kernel API with the instance
of 'struct device' in 'struct dw_mci' is then incorrect. Fix this by
converting the copy of 'struct device' in 'struct dw_mci' to a reference.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>