bindings: nordic,nrf-rtc.yaml: Do not require 'ppi-wrap' property

Looking at the code, this flag was probably made 'required: true' by
mistake. Combining 'type: boolean' with 'required: true' for 'ppi-wrap'
means that all nodes that use this binding are required to have a
'ppi-wrap;' property.

The mistake was hidden by a bug in edtlib (failing to flag missing
'required: true' booleans).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>