firmware: zynqmp: Do not bind PD driver in SPL if disabled

Change if condition to cover SPL flow. SPL needs to have
CONFIG_SPL_POWER_DOMAIN enabled to be able to bind
CONFIG_ZYNQMP_POWER_DOMAIN driver.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/8e1d381013a0ce39d736da166d2b401c4b12d38a.1646064792.git.michal.simek@xilinx.com