rockchip: sdhci: support 8bit bus-width

rockchip: sdhci: support 8bit bus-width

The Rockchip-specific SDHCI wrapper does not process the 'bus-width'
property in the SDHCI node. Consequently, the bus is always kept in
4bit mode, even if 8bit wide operation is available, supported and
requested in the DTS.

This change adds processing of the 'bus-width' property and sets the
host capability flag for an 8bit wide bus, if set to 8. As the logic
in sdhci.c does not support clearing the 4bit capability, we assume
that 4bit operation is always supported.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>