arm: Move systick device creation from NVIC to ARMv7M object

There's no particular reason why the NVIC should be owning the
SysTick device objects; move them into the ARMv7M container object
instead, as part of consolidating the "create the devices which are
built into an M-profile CPU and map them into their architected
locations in the address space" work into one place.

This involves temporarily creating a duplicate copy of the
nvic_sysreg_ns_ops struct and its read/write functions (renamed as
v7m_sysreg_ns_*), but we will delete the NVIC's copy of this code in
a subsequent patch.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20210812093356.1946-3-peter.maydell@linaro.org