tegra2: fix warning: "assert" redefined

tegra2: fix warning: "assert" redefined

Commit 21726a7 "Add assert() for debug assertions" caused build
warnings for all tegra2 based boards:

clock.c:36:1: warning: "assert" redefined
In file included from clock.c:29:
include/common.h:144:1: warning: this is the location of the previous definition

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>