Add new helper routines for sdhci bridge drivers that use gpio pins for
card presence and write protect switch detection.

Use the new sdhci_fdt_gpio helper functions to add full support for FDT
gpio pins for detecting card insert/remove and write protect for ti_sdhci.

Include sys/systm.h for use of bootverbose.

Revert to ti_sdhci driver's historic behavior: assume an sd card is
writable if the fdt data doesn't provide a gpio pin for reading the write
protect switch and also doesn't contain a "wp-disable" property.