powerpc/powernv: Skip registering log region when CONFIG_PRINTK=n

When CONFIG_PRINTK=n, log_buf_addr_get() returns NULL and log_buf_len_get()
return 0. Check for these return values and skip registering the dump buffer.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>