common: Add new clk command

Command provides just dump subcommand for showing clock
frequencies in a soc.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Stefano Babic <sbabic@denx.de>
diff --git a/README b/README
index 176de61..ba4be66 100644
--- a/README
+++ b/README
@@ -896,6 +896,7 @@
 		CONFIG_CMD_BSP		* Board specific commands
 		CONFIG_CMD_BOOTD	  bootd
 		CONFIG_CMD_CACHE	* icache, dcache
+		CONFIG_CMD_CLK   	* clock command support
 		CONFIG_CMD_CONSOLE	  coninfo
 		CONFIG_CMD_CRC32	* crc32
 		CONFIG_CMD_DATE		* support for RTC, date/time...