platform: openpiton: Move openpiton platform from fpga to generic

The OpenPiton framework has a generic PMU that is not used by OpenSBI.
Due to OpenSBI’s build system we cannot directly reuse the generic
platform functions, so move the OpenPiton platform to generic. Also due
to the generic platform is where new features are added.

Signed-off-by: Manuel Hernández Méndez <maherme.dev@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20250813104759.33276-1-maherme.dev@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>

docs/platform/generic.md

@@ -47,6 +47,7 @@ RISC-V Platforms Using Generic Platform
 * **SiFive HiFive Unleashed** (*[sifive_fu540.md]*)
 * **Spike** (*[spike.md]*)
 * **T-HEAD C9xx series Processors** (*[thead-c9xx.md]*)
+* **OpenPiton FPGA SoC** (*[fpga-openpiton.md]*)
 
 [andes-ae350.md]: andes-ae350.md
 [qemu_virt.md]: qemu_virt.md
@@ -55,3 +56,4 @@ RISC-V Platforms Using Generic Platform
 [sifive_fu540.md]: sifive_fu540.md
 [spike.md]: spike.md
 [thead-c9xx.md]: thead-c9xx.md
+[fpga-openpiton.md]: fpga-openpiton.md