ARC: make CONFIG_STANDALONE_LOAD_ADDR overridable

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
ARC: use "bootargs" from standard U-Boot environment

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
ARC: snps_xplorer configuration clean-up

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>