BCM270X_DT: Add sdio_overclock parameter to sdio overlay

The sdio_overclock parameter is like the overclock_50 parameter, i.e.
it sets an alternate frequency (in MHz) to use when the MMC framework
requests 50MHz, except that it applies to the SDIO bus.

Be aware that the actual frequencies achievable are limited to even integer
divisions of 250MHz, and that the driver will round up to include fractions
(e.g. 62 will include 62.5) but then round down to the nearest frequency.
In other words, the chosen frequency is the highest possible that is less than
the parameter value + 1. In practise this means that 62 is the only sensible
value.

Examples:
  250MHz/4 = 62.5MHz (sdio_overclock=62)
  250MHz/2 = 125MHz (sdio_overclock=125) # Too fast