arm64: dts: qcom: msm8939: Add clock-frequency for broadcast timer

Looks like not all firmware versions used for MSM8939 program the timer
frequency for both broadcast/MMIO timers, causing a WARNING at runtime:

WARNING: CPU: 0 PID: 0 at kernel/time/clockevents.c:38 cev_delta2ns+0x74/0x90
pc : cev_delta2ns+0x74/0x90
lr : clockevents_config.part.0+0x64/0x8c
Call trace:
 cev_delta2ns+0x74/0x90
 clockevents_config_and_register+0x20/0x34
 arch_timer_mem_of_init+0x374/0x534
 timer_probe+0x88/0x110
 time_init+0x14/0x4c
 start_kernel+0x2c0/0x640

Unfortunately there is no way to fix the firmware on most of these
devices since it's proprietary and signed. As a workaround, specify the
clock-frequency explicitly in the DT to fix the warning.

Fixes: 61550c6c

 ("arm64: dts: qcom: Add msm8939 SoC")
Reported-by: Vincent Knecht <vincent.knecht@mailoo.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20231204-msm8939-timer-v1-1-a2486c625786@gerhold.net
Signed-off-by: Bjorn Andersson <andersson@kernel.org>