target/ppc: Move mul{li, lw, lwo, hw, hwu} instructions to decodetree.

target/ppc: Move mul{li, lw, lwo, hw, hwu} instructions to decodetree.

Moving the following instructions to decodetree specification :
	mulli                   	: D-form
	mul{lw, lwo, hw, hwu}[.]	: XO-form

The changes were verified by validating that the tcg ops generated by those
instructions remain the same, which were captured with the '-d in_asm,op' flag.
Also cleaned up code for mullw[o][.] as per review comments while
keeping the logic of the tcg ops generated semantically same.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Chinmay Rath <rathc@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>