psci: Do not define do_poweroff() if CONFIG_SYSRESET_CMD_POWEROFF is enabled

CONFIG_SYSRESET_CMD_POWEROFF defines do_poweroff() in sysreset-uclass.c
that's why don't define it twice when both CONFIG_SYSRESET_CMD_POWEROFF and
CONFIG_CMD_POWEROFF are enabled. CONFIG_SYSRESET_CMD_POWEROFF depends on
CONFIG_CMD_POWEROFF.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>