fpga: intel-m10-bmc: Rework flash read/write

Access to flash staging area is different for N6000 from that of the
SPI interfaced counterparts. To make it easier to differentiate flash
access path, move read/write into new functions where the new access
path can be easily placed into. Rework the unaligned access such the
behavior it matches for both read and write.

This change also renames m10bmc_sec_write() to m10bmc_sec_fw_write() as
it would have a name conflict otherwise.

Co-developed-by: Tianfei zhang <tianfei.zhang@intel.com>
Signed-off-by: Tianfei zhang <tianfei.zhang@intel.com>
Co-developed-by: Russ Weight <russell.h.weight@intel.com>
Signed-off-by: Russ Weight <russell.h.weight@intel.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230116100845.6153-7-ilpo.jarvinen@linux.intel.com