arch: xtensa: clean up interrupt handling

Simplifying flow of handling interrupts:
- removing all _soc_inthandlers.h
- removing xtensa_intgen*
- removing XTENSA_GEN_HANDLERS Kconfig
- keeping optimized irq detection
- single handler with irq level as parameter

Signed-off-by: Maciej Kusio <rysiof@gmail.com>