Ramoops may be useful even without oftree support, as kernels
booted without a DT may have other means to reserve the
ramoops memory.

Fixes:
In function `ramoops_probe':
undefined reference to `of_add_reserve_entry'

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>