mmc: sdhi: use maximum width for the sdbuf register

Make use of the 64 bit sdbuf width on Renesas R-Car Gen3. If the
registers are 8 byte apart, the width is also 64 bit. For all others,
the width is 32 bit, even if the registers are only 16 bit apart.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>