armv7m_nvic: Implement ICSR without using internal GIC state

armv7m_nvic: Implement ICSR without using internal GIC state

Change the implementation of the Interrupt Control and State Register
in the v7M NVIC to not use the running_irq and last_active internal
state fields in the GIC. These fields don't correspond to state in
a real GIC and will be removed soon.
The changes to the ICSR are:
 * the VECTACTIVE field is documented as identical to the IPSR[8:0]
   field, so implement it that way
 * implement RETTOBASE via looking at the active state bits

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1438089748-5528-2-git-send-email-peter.maydell@linaro.org