ARC: bootm: Refactor GO and PREP subcommands implementation

Refactor GO and PREP subcommands implementation for a simpler
override in the boards platform code.

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