mfd: intel-m10-bmc: Change staging size to a variable

mfd: intel-m10-bmc: Change staging size to a variable

The secure update driver does a sanity-check of the image size in
comparison to the size of the staging area in FLASH. Instead of
hard-wiring M10BMC_STAGING_SIZE, move the staging size to the
m10bmc_csr_map structure to make the size assignment more flexible.

Co-developed-by: Russ Weight <russell.h.weight@intel.com>
Signed-off-by: Russ Weight <russell.h.weight@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Peter Colberg <peter.colberg@intel.com>
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240402184925.1065932-1-peter.colberg@intel.com
Signed-off-by: Lee Jones <lee@kernel.org>