reset: socfpga: add reset handling for old kernels

This adds code to take peripherals out of reset based on an environment
variable. This is in preparation for removing the code that does this from
SPL.

However, some drivers even in current Linux cannot handle peripheral reset,
so until this works, we need a compatibility workaround.

This workaround is implemented in the 'assert' and 'remove' callbacks of
this reset driver: the 'assert' callback does not disable peripherals that
were already taken out of reset, while the 'remove' callback, which is
called on OS_PREPARE, deasserts all peripheral resets if the environment
variable "socfpga_legacy_reset_compat" is set to 1, which is what the gen5
SPL did up to now.

This is in preparation to clean up the SPL and implementing proper reset
handling for U-Boot.

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

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