ARM: Make target_phys_addr_t 64 bits and physaddrs 40 bits

ARM: Make target_phys_addr_t 64 bits and physaddrs 40 bits

Make target_phys_addr_t 64 bits for ARM targets, and set
TARGET_PHYS_ADDR_SPACE_BITS to 40.  This should have no effect for ARM
boards where physical addresses really are 32 bits (except perhaps a
slight performance hit on 32 bit hosts for system emulation) but allows
us to implement the Large Physical Address Extensions for Cortex-A15,
which mean 40 bit physical addresses.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>