cleanup: soc: it8xxx2: remove unnecessary code

Code removed:
- IT8XXX2 doesn't support soc level software interrupt hence remove
  them.
- To use common macro to access csr (control status register).
- To remove CONFIG_RISCV_HAS_PLIC related code. IT8XXX2 uses its own
  interrupt controller code.
- To remove ite_write and ite_read. We don't use them anymore.

Code changed:
- Return true from arch_irq_is_enabled() when external interrupt-enable
  bit, and SOC's IER are both true.

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>