The current FWU_SMC_UPDATE_DONE implementation incorrectly passes
an unused framework cookie through to the 1st argument in the
platform function `bl1_plat_fwu_done`. The intent is to allow
the SMC caller to pass a cookie through to this function.

This patch fixes FWU_SMC_UPDATE_DONE to pass x1 from the caller
through to `bl1_plat_fwu_done`. The argument names are updated
for clarity.

Upstream platforms currently do not use this argument so no
impact is expected.

Change-Id: I107f4b51eb03e7394f66d9a534ffab1cbc09a9b2