powerpc/io: Unhook MMIO accessors

Now that PPC_INDIRECT_MMIO is removed, it's not possible/necessary
to hook any of the "memory" accessors, so turn them back into regular
static inlines, and restrict the hooking mechanism to the "pio"
accessors only.

Move the #defines that signal each routine is implemented next to the
implementation, and update the out-of-date comment.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218105523.416573-12-mpe@ellerman.id.au