Pico SDK defines `__dsb()` and `__sev()` so use those instead of the CMSIS
equivalents. This matches the use of `__wfi()` in lieu of `__WFI()` and
lowers the dependency on CMSIS headers.
And then, move the include of "RP2040.h" from the widely-included
"mphalport.h" to specific source files that need this header, to keep its
inclusion contained.
Signed-off-by: Phil Howard <phil@gadgetoid.com>