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>