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/xilinx/zynqmp/aarch64/zynqmp_common.c b/plat/xilinx/zynqmp/aarch64/zynqmp_common.c
index 0a878c3..6e5cee3 100644
--- a/plat/xilinx/zynqmp/aarch64/zynqmp_common.c
+++ b/plat/xilinx/zynqmp/aarch64/zynqmp_common.c
@@ -297,9 +297,9 @@
 	mmio_write_32(IOU_SCNTRS_CONTROL, IOU_SCNTRS_CONTROL_EN);
 }
 
-uint64_t plat_get_syscnt_freq(void)
+unsigned long long plat_get_syscnt_freq(void)
 {
-	uint64_t counter_base_frequency;
+	unsigned long long counter_base_frequency;
 
 	/* FIXME: Read the frequency from Frequency modes table */
 	counter_base_frequency = zynqmp_get_system_timer_freq();