sunxi: mmc: Move header to the driver directory

The MMC controller driver is (and ought to be) the only user of these
register definitions. Put them in a header next to the driver to remove
the dependency on a specific ARM platform's headers.

Due to the sunxi_mmc_init() prototype, the file was not renamed. None of
the register definitions were changed.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>