arch: arc: irq: bugs fix for fast irq in one register bank config

* The stack pointer (SP) register points to the lowest-used address of
   a downward-growing stack, so memory address [sp] is used, we can't
   modify it.

* In firq_no_switch case, we need to pop sp, which pushed before
   _isr_demux function in firq_nest function.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>