sfc: extend NVRAM MCDI handlers

sfc: extend NVRAM MCDI handlers

Support variable write-alignment, and background updates.  The latter
 allows other MCDI to continue while the device is processing an
 MC_CMD_NVRAM_UPDATE_FINISH, since this can take a long time owing to
 e.g. cryptographic signature verification.
Expose these handlers in mcdi.h, and build them even when
 CONFIG_SFC_MTD=n, so they can be used for devlink flash in a
 subsequent patch.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/de3d9e14fee69e15d95b46258401a93b75659f78.1739186253.git.ecree.xilinx@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>