arc: implement new ABI for starting kernel

 * for passing command-line r0 = 1, r2 = pointer to command line
 * for passinf FDT r0 = 2, r2 = pointer do FDT blob

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
xplorer: enable support of FDT

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
arcangel4: enable support of FDT

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
arc: add arch-specific flag used for detection of simulation

Will be used at least in ARC Serial driver to work-around ISS bug

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
arc: detect if running in simulation

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
serial/serial_arc: add work-around of ISS bug

Explanation is in in-lined comment.

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