MIPS: Alchemy: Add return value check for strict_strtoul()

arch/mips/alchemy/devboards/prom.c: In function 'prom_init':
arch/mips/alchemy/devboards/prom.c:60: error: ignoring return value of
'strict_strtoul', declared with attribute warn_unused_result

Signed-off-by: Yoichi Yuasa <yuasa@linux-mips.org>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/1761/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>