clocksource/drivers/arm_global_timer: Fix Kconfig "its" grammar

clocksource/drivers/arm_global_timer: Fix Kconfig "its" grammar

Use the possessive "its" instead of the contraction "it's"
where appropriate.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20220715015852.12523-1-rdunlap@infradead.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>