octeon_ep: Add missing check for ioremap

Add check for ioremap() and return the error if it fails in order to
guarantee the success of ioremap().

Fixes: 862cd659

 ("octeon_ep: Add driver framework and device initialization")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Link: https://lore.kernel.org/r/20230615033400.2971-1-jiasheng@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>