command: Fix SMC and HVC maximum number of arguments

smc and hvc commands take upto 8 user input arguments, the maximum
number of arguments of the U_BOOT_CMD macro should set to 9.

Besides, fix the typo (arg7 -> arg6) in hvc command's help message.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>