Move `plat_get_syscnt_freq()` to arm_common.c

This patch moves the definition for `plat_get_syscnt_freq()`
from arm_bl31_setup.c to arm_common.c. This could be useful
in case a delay timer needs to be installed based on the
generic timer in other BLs.
This patch also modifies the return type for this function
from `uint64_t` to `unsigned long long` within ARM and other
platform files.

Change-Id: Iccdfa811948e660d4fdcaae60ad1d700e4eda80d
diff --git a/plat/nvidia/tegra/soc/t210/plat_setup.c b/plat/nvidia/tegra/soc/t210/plat_setup.c
index 3fce8a2..eecedb3 100644
--- a/plat/nvidia/tegra/soc/t210/plat_setup.c
+++ b/plat/nvidia/tegra/soc/t210/plat_setup.c
@@ -80,7 +80,7 @@
 /*******************************************************************************
  * Handler to get the System Counter Frequency
  ******************************************************************************/
-uint64_t plat_get_syscnt_freq(void)
+unsigned long long plat_get_syscnt_freq(void)
 {
 	return 19200000;
 }