board: novena: Add code to disable peripherals

Prior device tree files would disable all devices, and then U-Boot
would enable them by changing their status from "disabled" to "okay".

Keep that code, but add the ability to disable devices as well by simply
deleting their device tree node.  Note that we can't change the status
from "okay" to "disabled" since the U-Boot "fdt" command is unable to
make strings longer, only shorter, and "disabled" is longer than "okay".

Signed-off-by: Sean Cross <xobs@kosagi.com>