MIPS: Alchemy: fix xxs1500 build error

This fixes:
alchemy/xxs1500/init.c: In function 'prom_init':
alchemy/xxs1500/init.c:57:17: error: ignoring return value of 'kstrtoul', declared with attribute warn_unused_result

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/2340/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>