fsi: Use of_property_read_reg() to parse "reg"

Use the recently added of_property_read_reg() helper to get the
untranslated "reg" address value.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20230609183056.1765183-1-robh@kernel.org


Signed-off-by: Joel Stanley <joel@jms.id.au>