target/hppa: Reduce TARGET_PHYS_ADDR_SPACE_BITS to 40

This is the value that is supported by both PA-8500 and Astro.
If we support a larger address space than expected, we trip up
software that did not fill in all of the page table bits,
expecting them to be ignored.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>