target/mips: Move physical addressing code to sysemu/physaddr.c

Declare get_physical_address() with local scope and move it along
with mips_cpu_get_phys_page_debug() to sysemu/physaddr.c new file.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-18-f4bug@amsat.org>