scripts: gen_syscalls: add missing --split-type case

With CONFIG_TIMEOUT_64BIT it is both k_timeout_t and k_ticks_t that
need to be split, otherwise many syscalls returning a number of ticks
are being truncated to 32 bits.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
(cherry picked from commit 2cdac33d)