hw/intc/arm_gic: Add Interrupt Group Registers

The Interrupt Group Registers allow the guest to configure interrupts
into one of two groups, where Group0 are higher priority and may
be routed to IRQ or FIQ, and Group1 are lower priority and always
routed to IRQ. (In a GIC with the security extensions Group0 is
Secure interrupts and Group 1 is NonSecure.)
The GICv2 always supports interrupt grouping; the GICv1 does only
if it implements the security extensions.

This patch implements the ability to read and write the registers;
the actual functionality the bits control will be added in a
subsequent patch.

Signed-off-by: Fabian Aggeler <aggelerf@ethz.ch>
Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1430502643-25909-5-git-send-email-peter.maydell@linaro.org
Message-id: 1429113742-8371-7-git-send-email-greg.bellows@linaro.org
[PMM: bring GIC_*_GROUP macros into line with the others, ie a
 simple SET/CLEAR/TEST rather than GROUP0/GROUP1;
 utility gic_has_groups() function;
 minor style fixes;
 bump vmstate version]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
