hw/arm/virt: Provide flash devices for boot ROMs

Add two flash devices to the virt board, so that it can be used for
running guests which want a bootrom image such as UEFI. We provide
two flash devices to make it more convenient to provide both a
read-only UEFI image and a read-write place to store guest-set
UEFI config variables. The '-bios' command line option is set up
to provide an image for the first of the two flash devices.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
hw/arm/virt: Provide PL031 RTC

UEFI mandates that the platform must include an RTC, so provide
one in 'virt', using the PL031.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
fix nwfpe case
unused cpsr
add mask helper