Make all available NVIC registers accessible

According to the ARMv7-M Technical Reference Manual[1], there are 124
IPR registers available on ARMv7-M, and 16 of all others. I don't know
where the original numbers came from, since on ARMv6-M, there are only 8
IPR registers available, and 1 of each of the others.[2]

This commit removes some test cases that were checking the address of
the last register. Since the last register has changed, those are no
longer applicable. I decided to remove instead of update them, since
they only really test the length of each register type, which is obvious
enough from the code.

[1]: https://silver.arm.com/download/ARM_and_AMBA_Architecture/AR580-DA-70000-r0p0-05rel0/DDI0403E_B_armv7m_arm.pdf
[2]: https://silver.arm.com/download/ARM_and_AMBA_Architecture/AR585-DA-70000-r0p0-00rel0/DDI0419C_arm_architecture_v6m_reference_manual.pdf