scripts: build: gen_isr_tables: fix calculation of THIRD_LVL_INTERRUPTS

The calculation of `THIRD_LVL_INTERRUPTS` bitmask in the
`update_masks()` function is wrong, the number of bits to shift
should be the sum of the first two levels.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
(cherry picked from commit 56570cc8)