[EMULATORS] arm_sysregs: Use ref_24mhz for 24mhz counter

We have dedicated referece timestamp for 24mhz counter so use
that for emulating 24mhz counter read instead of 100hz reference
timestamp.

This patch is more of a typo-fix and does not change the behaviour
of 24mhz counter in anyway.

Signed-off-by: Anup Patel <anup@brainfault.org>