ARC: [plat-hsdk] Increase SDIO CIU frequency to 50000000Hz

With current SDIO CIU clock frequency (12500000Hz) DW MMC
controller fails to initialize some SD cards (which don't
support slow mode).

So increase SDIO CIU frequency from 12500000Hz to 50000000Hz by
switching from the default divisor value (div-by-8) to the
minimum possible value of the divisor (div-by-2) in HSDK platform
code.

Reported-by: Vineet Gupta <vgupta@synopsys.com>
Tested-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>