board: axs10x - cap max SDIO clock value to bus/2

board: axs10x - cap max SDIO clock value to bus/2

It turned out with some boards (FPGA firmwares?) and cards combos
current clock settings doesn't work as expected leading to strange
card freezes or corrupted data being read from the card.

Especially this was seen with Transcend 2Gb cards shipped as a part of
ARC SDP:
----------------->8---------------
AXS# mmcinfo
Device: Synopsys Mobile storage
Manufacturer ID: 74
OEM: 4a60
Name: SDC
Tran Speed: 50000000
Rd Block Len: 512
SD version 3.0
High Capacity: No
Capacity: 1.8 GiB
Bus Width: 4-bit
Erase Group Size: 512 Bytes
AXS# fatload mmc 0
** Unrecognized filesystem type **
----------------->8---------------

With this change that problem is fixed.
Note "Tran Speed" above doesn't match clock value set in DW MMC.
It is max value for card's speed class.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>