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some specific board need to disable/enable specific gpio when
suspend/resume, so we add this function, bootloader can pass the
specific gpio, and we can handle these gpios in bl31 suspend/resuem
function.

Change-Id: I373b03ef9202ee4a05a2b9caacdfa01b47ee2177

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.