irqchip/aspeed-scu-ic: Refactor driver to support variant-based initialization

The SCU IC driver handles each AST2600 instance with separate
initialization functions and hardcoded register definitions, which is
inflexible and creates duplicated code.

Consolidate the implementation by introducing a variant-based structure,
selected via compatible string, and use a unified init path and MMIO access
via of_iomap().  This simplifies the code and prepares for upcoming SoCs
like AST2700, which require split register handling.

[ tglx: Cleaned up coding style and massaged change log ]

Signed-off-by: Ryan Chen <ryan_chen@aspeedtech.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250908011812.1033858-2-ryan_chen@aspeedtech.com