riscv: dts: mpfs: migrate to OF_UPSTREAM

riscv: dts: mpfs: migrate to OF_UPSTREAM

The U-Boot copy of the mpfs devicetree has, in general, been neglected
somewhat in comparison to the one in Linux. Moving to OF_UPSTREAM to
keep both in sync should serve to eliminate that discrepancy.

Additionally, moving to OF_UPSTREAM will let U-Boot automatically pick
up the devicetree rework that is in progress at [1].

Link: https://lore.kernel.org/all/20241002-private-unequal-33cfa6101338@spud/
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>