mmc: mv_sdhci: fix 8bus width access for 88SV331xV5

Marvell 88SV331xV5 platform's sdhci host control is not very standard
with the spec in the 8bit handling. It need to set its private register
to switch to the 8bit mode which is not included in the standard sdhci
registers.

This patch mainly hacks the writeb method, and set its private register
if it find the driver is going to switch to the 8bit mode.

Signed-off-by: Lei Wen <leiwen@marvell.com>