board: AXS10x - update SDIO clock value

board: AXS10x - update SDIO clock value

With the most recent board firmware correct SDIO clock is 50MHz as
opposed to 25 MHz before.

Also set max frequency of MMC data exchange equal to SDIO clock -
because there's no way to transfer data faster than interface clock.

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