power: bcl_peripheral: Update optional devicetree property read macro

BCL peripheral probe throws error if an optional devicetree property
is not defined for that target. Fix optional devicetree node read
macro so that it doesn't throw any error if property is not defined.

Change-Id: I76c052ede0b396ad0ab6e8335b4ae2b09cffee6e
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>