arm: socfpga: gen5: deassert peripheral reset by default

To keep the current behaviour of taking all peripherals out of reset
before booting the OS before removing that code from socfpga gen5 SPL,
this enables the new behaviour by default for all gen5 boards by adding
the environment variable "socfpga_legacy_reset_compat=1" to the default
environment.

This can be overridden in board config files or by saving an environment
without this variable enabled.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>