This gives the SDHCI specific registers a common name and moves
them to a separate file for use by other SDHCI like drivers.

The SDHCI spec has some 16bit and 8bit registers. The i.MX accesses
these as 32bit registers. These register defines now are named after
the SDHCI registers they combine into one, for example:

SDHCI_HOST_CONTROL__POWER_CONTROL__BLOCK_GAP_CONTROL

is the 32bit version of the SDHCI registers HOST_CONTROL, POWER_CONTROL
and BLOCK_GAP_CONTROL.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>