xilinx: Enable OF_BOARD for zynq and zynqmp boards

The commit 98550343 ("fdt: Don't call board_fdt_blob_setup() without
OF_BOARD") forced to enable OF_BOARD for platforms which provide DT
externally. Zynq/ZynqMP boards are using this feature for a long time
that's why there is a need to enable it by default.

Also code expects to return error in case of error that's why also fill it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/9f11bbffe2849f4da7d72712082d579262fe8fd8.1645104518.git.michal.simek@xilinx.com