cc32xx: Redefine CMSIS IRQn_Type enum to unsigned int

Previously, calling NVIC_SetPriority(IRQn_Type irqn, ....) with
the NWP interrupt number of 171 caused a hard fault during a
subsequent svc #0 instruction during _Swap().

GNU compiler is generating a bit extension instruction (sxtb) which
converts a positive IRQ value argument to a negative value when
casting to the CMSIS IRQn_Type enum parameter type.

This generates a negative index, which then writes to an SCB
control register instead of NVIC register, causing a hard
fault later on.

This issue only occurs when passing interrupt numbers > 0x80
(eg: 171 (0xab) for the NWP) to the CMSIS NVIC apis.

The solution here is simply to redefine IRQn_Type to be an
unsigned 32 bit integer, while redefining the CMSIS IRQn_Type
enum definitions for interrupts less than zero.

Jira: ZEP-1958

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>