ARC: Switch to generic accessors

First of all U-Boot is not that performance oriented as real run-time
software like OS or user bare-metal app so we may afford being not super
fast as we only being executed once. That in return allows us to be more
universal and support wider variety of devices.

And looking forward that will significantly reduce maintenance and simplify
support of newer architectures.

And while at it we add quad-word accessors like readq(), writeq() etc.

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