kconfig: move CONFIG_SYS_HZ to lib/Kconfig

CONFIG_SYS_HZ is always defined as 1000 in config_fallbacks.h
(but some boards still have redundant definitions).

This commit moves the definition and the document in README to
Kconfig.  Since lib/Kconfig can assure that CONFIG_SYS_HZ is 1000,
the sanity check in lib/time.c should be removed.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/lib/Kconfig b/lib/Kconfig
index 88e5da7..a889c22 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -8,4 +8,12 @@
 
 	  If unsure, say N.
 
+config SYS_HZ
+	int
+	default 1000
+	help
+	  The frequency of the timer returned by get_timer().
+	  get_timer() must operate in milliseconds and this option must be
+	  set to 1000.
+
 endmenu