fu540: prci: use common reset indexes defined in binding header

Indexes of reset signals available in PRCI driver are also
defined in include/dt-bindings/reset/sifive-fu540-prci.h.
So use those instead of defining new ones again within the
fu540-prci driver.

Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>