[ot] hw/opentitan: ot_spi_device: implement Passthrough mode

This commit implements the Passthrough mode on SPI Device. Passthrough
mode allows the device to act as a proxy for a downstream flash device,
optionally intercepting commands to send back its own values, filtering
commands sent downstream, or transparently translating address and
payload bytes.

This also "implements" the Disabled mode, which discards all bytes sent
to the device.

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

Author: ziuziakowska