[EMULATORS] arm_sysregs: Add emulation of VExpress cfgctrl registers

The VExpress-A9 and VExpress-A15 guests have clocking and voltage regulation
vial cfgctrl registers of arm_sysregs. Recent linux kernels (i.e. linux-3.10
or higher) use cfgctrl registers of VExpress boards extensively hence this
patch improves our arm_sysregs emulator to provide correct emulation of
cfgctrl registers.

Signed-off-by: Anup Patel <anup@brainfault.org>