Prevent wraparound in SPRAM data access

[mchack-work]

QEMU and real hardware behave differently if you access RAM. If you access higher than 0x4001ffff real hardware wraps around to the beginning of RAM.

[secworks]

Added an experimental filter to the RAM access in branch https://github.com/tillitis/tillitis-key1/tree/match_qemu_ram_mem
The fix needs testing from SW. The expected behaviour now is that 0xdeadbeef is returned for addresses larger than max ram, but within the RAM prefix.

[secworks]

Ready for testing, se PR #160

[dehanj]

Confirming that QEMU detects a read/write outside of RAM as an invalid operation and rejects it. Does seem as it tries to reset, and gets stuck trying to access FW in app-mode.

Invalid write at addr 0x4011FF00, size 4, region '(null)', reason: rejected
tk1_mmio_write: bad write: addr=0xd00007fc size=4 val=0x60 msg='write to FW_RAM in app-mode'
tk1_mmio_write: bad write: addr=0xd00007f8 size=4 val=0x0 msg='write to FW_RAM in app-mode'
tk1_mmio_write: bad write: addr=0xd00007f4 size=4 val=0x0 msg='write to FW_RAM in app-mode'
tk1_mmio_write: bad write: addr=0xd00007f0 size=4 val=0x0 msg='write to FW_RAM in app-mode'
tk1_mmio_write: bad write: addr=0xd00007ec size=4 val=0x0 msg='write to FW_RAM in app-mod