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We add plat parameter structs to support BL2 to pass variable-length,
variable-type parameters to BL31. The parameters are structured as a
link list. During bl31 setup time, we travse the list to process each
parameter. throuth this way, we can get the reset or power off gpio
parameter, and do hardware control in BL31. This structure also can
pass other parameter to BL31 in future.