sh: Relax inline assembly constraints

When dereferencing the memory address contained in a register and
modifying the value at that memory address, the register should not be
listed in the inline asm outputs. The value at the memory address is an
output (which is taken care of with the "memory" clobber), not the register.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>