board: mpfs_icicle: implement board_fdt_blob_setup()/board_fit_config_name_match()

The firmware on the Icicle is capable of providing a devicetree in a1 to
U-Boot, but until now the devicetree has been packaged in a "payload" [1]
alongside U-Boot (or other bootloaders/RTOSes) and appended to the image.
The address of this appended devicetree is placed in a1 by the firmware.
This meant that the mechanism used by OF_SEPARATE to locate the
devicetree at the end of the image would pick up the one provided by the
firmware when u-boot-nodtb.bin was in the payload and U-Boot's devicetree
when u-boot.bin was.

The firmware is now going to be capable of providing a minimal devicetree
(quite cut down due to severe space constraints), but this devicetree is
linked into the firmware that runs out of the L2 rather than at the end
of the U-Boot image.

Implement board_fdt_blob_setup() so that this devicetree can be
optionally used, and the devicetree provided in the "payload" can be
used without relying on "happening" to implement the same strategy as
OF_SEPARATE expects in combination with u-boot-nodtb.bin.
Unlike other RISC-V boards, the firmware provided devicetree is only
used when OF_BOARD is set, so that the almost certainly more complete
devicetree in U-Boot will be used unless explicitly requested otherwise.

Implement board_fit_config_name_match(), so that, using the firmware
provided cut-down/minimal dtb, U-Boot can select one of several
devicetrees when MULTI_DTB_FIT is enabled.

Enabling both MULTI_DTB_FIT and OF_BOARD will lead to a conflict
between the two options, with the latter taking priority due to
board_fdt_blob_setup() being executed before board_fit_config_name_match(),
which causes gd->fdt_blob to be overwritten with a pointer to the
minimal devicetree rather than the location of the fit image containing
the multiple dtbs. Let MULTI_DTB_FIT take priority in this case, by
explicitly blocking the override when MULTI_DTB_FIT is enabled.

Link: https://github.com/polarfire-soc/hart-software-services/blob/master/tools/hss-payload-generator/README.md [1]
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Leo Yu-Chi Liang <ycliang@andestech.com>

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