mmc: sdhi: Add r8a7795 support

Registers are 64bit apart, so we refactor bus_shift handling a little and set
it based on the DT compatible. Also, EXT_ACC is different. It has been tested
on a Salvator-X (Gen3) and, to check for regressions, on a Lager (Gen2).

Signed-off-by: Ai Kyuse <ai.kyuse.uw@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>