arm: socfpga: sr1500: Misc updates (SPI speed, env location)

This patch makes the following changes to the SR1500 board port:

- Update defconfig to support SPI NOR (use make savedefconfig).
- Increase SPI speed to a maximum of 100MHz for faster system
  bootup.
- Change environment location, so that its not between SPL and
  main U-Boot. This way the combined SPL / U-Boot image can
  be used for updates.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>