mmc: sdhci: Enable 8-bit bus width only for 3.0 spec onwards

CAP register don't have any information for 8-bit buswidth support
on 2.0 sdhci spec, only from 3.0 onwards bit[18] got this information.

Due to this misassignment in sdhci, mmc is setting 8-bit buswidth using
mmc_set_bus_width even if controller doesn't support.
Below change has code information.
"mmc: Properly determine maximum supported bus width"
(sha1: 7798f6db)

Bug log: <mmc plus and emmc cards)
-------
zynq-uboot> mmcinfo
Error detected in status(0x208100)!
Device: zynq_sdhci
Manufacturer ID: fe
.....

So enable 8-bit support only for 3.0 spec using CAP and for below 3.0
assign mmc->host_caps = MMC_MODE_8BIT on respective platform driver
if host have a support.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>