ARM: socfpga: add fpga build and bsp handoff instructions to readme

This patch adds the steps to manually (re)build a Quartus FPGA project,
generate the required BSP glue, and update u-boot handoff files for
mainline SPL support. Requires Quartus toolchain and current U-Boot.

Signed-off-by: Steve Arnold <stephen.arnold42@gmail.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>