Raise the SDHCI timeout to 10 seconds and add a sysctl to allow changing
this value at runtime.

The SD card specification says that a block write or a block erase can take
up to 250ms to complete and thus, under some circumstances, the existent 2
seconds timeout was triggering with normal usage.

This change fixes the sporadic controller timeout that happens on RPi and
RPi 2.

Discussed with: ian (some time ago)