zynq: load u-boot-dtb.img if CONFIG_OF_SEPARATE is defined

zynq: load u-boot-dtb.img if CONFIG_OF_SEPARATE is defined

SPL should load "u-boot-dtb.img" if both CONFIG_OF_CONTROL
and CONFIG_OF_SEPARATE are defined.
Otherwise, "u-boot.img" should be loaded.

Since CONFIG_OF_CONTROL is always undefined for SPL_BUILD,
the undef block should be moved below the conditional definition
of CONFIG_SPL_FAT_LOAD_PAYLOAD_NAME.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>