MIPS: fix mips_cache fallback without __builtin_mips_cache

The "R" constraint supplies the address of an variable in a register. Use
"r" instead and adjust asm to supply the content of addr in a register
instead.

Fixes: 2b8bcc5a ("MIPS: avoid .set ISA for cache operations")
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>