post/lib_powerpc/multi.c: fix stack overflow error

post/lib_powerpc/multi.c: fix stack overflow error

The code and comment disagreed: the comment claimed that r6...r31
were copied, and consequently the arrays for "src" and "dst" were
declared with 26 entries, but the actual code ("lmw r5,0(r3)" and
"stmw r5,0(r4)") copied _27_ words (r5 through r31), which resulted
in false "POST cpu Error at multi test" messages.

Fix the comment and the array sizes.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Kim Phillips <kim.phillips@freescale.com>
Cc: Andy Fleming <afleming@gmail.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Anatolij Gustschin <agust@denx.de>