doc(spi_device): Add WEL description
doc(spi_device): Add WEL description

This commit adds a description to WEL in the FLASH_STATUS CSR. SW and HW
both may modify the field.

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

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