nrf51: Fix last GPIO CNF address

nrf51: Fix last GPIO CNF address

NRF51_GPIO_REG_CNF_END doesn't actually refer to the start of the last
valid CNF register: it's referring to the last byte of the last valid
CNF register.

This hasn't been a problem up to now, as current implementation in
memory.c turns an unaligned 4-byte read from 0x77f to a single byte read
and the qtest only looks at the least-significant byte of the register.

But when running with patches which fix unaligned accesses in memory.c,
the qtest breaks.

Considering NRF51 doesn't support unaligned accesses, the simplest fix
is to actually set NRF51_GPIO_REG_CNF_END to the start of the last valid
CNF register: 0x77c.

Now, qtests work with or without the unaligned access patches.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cameron Esfahani <dirty@apple.com>
Message-id: 51b427f06838622da783d38ba56e3630d6d85c60.1586925392.git.dirty@apple.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>