ppc: Improve emulation of THRM registers

The 75x and 74xx processors have some thermal monitoring SPRs that
some OSes such as MacOS do use. Our current "dumb" implementation
isn't good enough and will cause some versions of MacOS to hang during
boot.

This lifts an improved emulation from MacOnLinux and adapts it to
qemu, thus fixing the problem.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[dwg: Fixed typo in comment, a number of minor checkpatch warnings,
 and a compile failure with CONFIG_USER_ONLY]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
