mmc: sdhci-cadence: use bitfield access macros for cleanup

Accessing register fields generally need mask and shift part.
Defining them separately, like SDHCI_CDNS_HRS06_TUNE_{SHIFT,MASK},
is tedious.

Register fields can be always defined by GENMASK (or, BIT if it it
a single bit).  They are nicely handled by FIELD_* macros.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>