microblaze: Wire generic xilinx board_late_init_xilinx()

Call generic board_late_init_xilinx() to be aligned with the rest of xilinx
platforms. Also getting rid of initrd_high/fdt_high and use
bootm_low/boot_size instead.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>