lib: utils/fdt: Add fdt_parse_timebase_frequency() function

We add fdt_parse_timebase_frequency() function which can be used
by ACLINT mtimer driver and platform code to get timebase frequency.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Xiang W <wxjstz@126.com>

include/sbi_utils/fdt/fdt_helper.h

@@ -50,6 +50,8 @@ int fdt_parse_hart_id(void *fdt, int cpu_offset, u32 *hartid);
 
 int fdt_parse_max_hart_id(void *fdt, u32 *max_hartid);
 
+int fdt_parse_timebase_frequency(void *fdt, unsigned long *freq);
+
 int fdt_parse_gaisler_uart_node(void *fdt, int nodeoffset,
 				struct platform_uart_data *uart);