mmc: sdhci-cadence: override spec version

The datasheet of the IP (sd4hc) says it is compiatible with SDHCI v4,
but the spec version field in the version register is read as 2
(i.e. SDHCI_SPEC_300) based on the RTL provided by Cadence.

Socionext did not fix it up when it integrated the IP into the SoCs.
So, it is working as SDHCI v3.

It is not a real problem because there is no difference in the program
flow in sdhci.c between SDHCI_SPEC_300/400, but set the real version
just in case.

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>