arch: common: sw_isr_common: Move table index computing logic to function

Since the shared IRQ code will also use the same logic to compute
the _sw_isr_table index, move the computing logic to a separate
function: z_get_sw_isr_table_idx(), which can be used by other
modules.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>