soc: nrf53: fix building anomaly 168 workaround

With GCC 12.3 and binutils 2.40, the build fails with:

<...>/zephyr/arch/arm/core/cortex_m/cpu_idle.S: Assembler messages:
<...>/zephyr/arch/arm/core/cortex_m/cpu_idle.S:51: Error: junk at end of line, first unrecognized character is `n'
<...>/zephyr/arch/arm/core/cortex_m/cpu_idle.S:133:   Info: macro invoked from here

Because the SOC_ON_EXIT_CPU_IDLE macro puts all the statements on a
single line, there must be a semicolon after .rept

Signed-off-by: Ben Wolsieffer <benwolsieffer@gmail.com>