[spi_device] Add flash/passthrouh modes in CFG.MODE
[spi_device] Add flash/passthrouh modes in CFG.MODE

Flash and Passthrough modes are added into CFG.MODE CSR.

Signed-off-by: Eunchan Kim <cb7bc7ceb22698f298c67faa994fd9ddf3cab298@opentitan.org>

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