dt-bindings: prci: add indexes for reset signals available in prci

Add bit indexes for reset signals within the PRCI module
on FU540-C000 SoC.
The DDR and ethernet sub-system's have reset signals
indicated by these reset indexes.

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>