sdhci_fsl_fdt: Add full support for software reset

When performing software reset, this controller does not clear all the
required hw registers. In particular, tuning block is left in enabled
state, inhibiting operation of some eMMC cards. The existing solution
was to disable the ability to call SDHCI_RESET_ALL.

As this issue is now better understood, enable the SDHCI_RESET_ALL flag,
provide a custom reset devmethod and clear selected registers by hand.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differential revision: https://reviews.freebsd.org/D32705