arm: zynq: spl: fix FPGA initialization

arm: zynq: spl: fix FPGA initialization

commit 4aba5fb857c1 ("arm: zynq: Rework FPGA initialization")
moved FPGA initialization from board_init() to arch_early_init_r(),
which is not called as part of the SPL

Fix this by calling arch_early_init_r() in the spl_board_init()
function, so the FPGA is correctly initialized

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>