[dif/spi_device] Update checklist and milestone
[dif/spi_device] Update checklist and milestone

This updates the DIF checklist and milestone for the SPI Device IP to
reflect the current state of the SW.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan