xplorer: remove obsolete address translation

With recent FPGA images (at least starting from RC3) CREGs are
automatically set and NAND controller sees DDR memory in the same
aperture as CPU.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
arc: preserve r25 from usage by compiler

Now compiler won't use r25 and we're sure it always points to global
data

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