ARC: Remove unused prerequisites from README

Readme mentioned some prerequisites which are in fact unused. This patch
removes mention of them. It also changes instructions to configure as
now libftdi is not used.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
ARC: Update target description scripts

This patch extracts description of Digilent HS interface from each
target script, and places in separate file. This file is sourced by each
script.

Interface is changed from ft2232 to ftdi.

Increase JTAG speed to 10 MHz from 7 MHz. EM Starter Kit 1.0 supports at
least 12 MHz, so I think it is safe to assume 10 MHz will work in most
cases.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>