arc: add Arcangel4 board support

Arcangel4 is a FPGA-based development board that is used for prototyping and
verificationof of both ARC hardware (CPUs) and software running upon CPU.

This board avaialble in 2 flavours:
 * Little-endian (arcangel4)
 * Big-endian (arcangel4-be)

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

Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Francois Bedard <fbedard@synopsys.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>