arc: Introduce a possibility to not relocate U-boot

Disabling relocation might be useful on ARC for 2 reasons:
 a) For advanced debugging with Synopsys proprietary MetaWare debugger
    which is capable of accessing much more specific hardware resources
    compared to gdb. For example it may show contents of L1 and L2 caches,
    internal states of some hardware blocks etc.

    But on the downside MetaWare debugger still cannot work with PIE.
    Even though that limitation could be work-arounded with change of ELF's
    header and stripping down all debug info but with it we won't have
    debug info for source-level debugging which is quite inconvenient.

 b) Some platforms which might benefit from usage of U-Boot basically
    don't have enough RAM to accommodate relocation of U-Boot so we
    keep code in flash and use as much of RAM as possible for more
    interesting things.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: York Sun <york.sun@nxp.com>
Cc: Stefan Roese <sr@denx.de>