x86: quark: MRC: Remove U_BOOT_DATE and U_BOOT_TIME from debug log

U_BOOT_DATE and U_BOOT_TIME are updated on every run of make command.
Therefore mrc.c file is recompiled every time when running make which means
that whole U-Boot binary is recompiled on every run of make command.

Simplify it and do not recompile U-Boot binary on every run of make command
by not depending on macros U_BOOT_DATE and U_BOOT_TIME.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>