irqchip/thead-c900-aclint-sswi: Generalize aclint-sswi driver and add MIPS P800 support

irqchip/thead-c900-aclint-sswi: Generalize aclint-sswi driver and add MIPS P800 support

Refactor the Thead specific implementation of the ACLINT-SSWI irqchip:

 - Rename the source file and related details to reflect the generic nature
   of the driver

 - Factor out the generic code that serves both Thead and MIPS variants.
   This generic part is compliant with the RISC-V draft spec [1]

 - Provide generic and Thead specific initialization functions

Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev@mobileye.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250612143911.3224046-5-vladimir.kondratiev@mobileye.com
Link: https://github.com/riscvarchive/riscv-aclint