dt-bindings: mmc: sdhci-st: Mention the discretionary "icn" clock

The interconnect (ICN) clock is required for functional working of
MMC on some ST platforms.  When not supplied it can result in
broken MMC and the following output:

        [   13.916949] mmc0: Timeout waiting for hardware interrupt.
        [   13.922349] sdhci: =========== REGISTER DUMP (mmc0)===========
        [   13.928175] sdhci: Sys addr: 0x00000000 | Version:  0x00001002
        [   13.933999] sdhci: Blk size: 0x00007040 | Blk cnt:  0x00000001
        [   13.939825] sdhci: Argument: 0x00fffff0 | Trn mode: 0x00000013
        [   13.945650] sdhci: Present:  0x1fff0206 | Host ctl: 0x00000011
        [   13.951475] sdhci: Power:    0x0000000f | Blk gap:  0x00000080
        [   13.957300] sdhci: Wake-up:  0x00000000 | Clock:    0x00003f07
        [   13.963126] sdhci: Timeout:  0x00000004 | Int stat: 0x00000000
        [   13.968952] sdhci: Int enab: 0x02ff008b | Sig enab: 0x02ff008b
        [   13.974777] sdhci: AC12 err: 0x00000000 | Slot int: 0x00000000
        [   13.980602] sdhci: Caps:     0x21ed3281 | Caps_1:   0x00000000
        [   13.986428] sdhci: Cmd:      0x0000063a | Max curr: 0x00000000
        [   13.992252] sdhci: Host ctl2: 0x00000000
        [   13.996166] sdhci: ADMA Err: 0x00000000 | ADMA Ptr: 0x7c048200
        [   14.001990] sdhci: ===========================================
        [   14.009802] mmc0: Got data interrupt 0x02000000 even though no data operation was in progress.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>