MIPS: Fix compile warnnings in kexec-elf-mips.c

Fix the following warnings:

kexec/arch/mips/kexec-elf-mips.c:161:41: warning: passing argument 3 of
‘dtb_set_initrd’ makes integer from pointer without a cast
dtb_set_initrd(&dtb_buf, &dtb_length, initrd_buf, initrd_buf + initrd_size);
                                         ^
In file included from kexec/arch/mips/kexec-elf-mips.c:33:0:
kexec/arch/mips/../../dt-ops.h:6:5: note: expected ‘off_t’ but argument is
of type ‘char *’
int dtb_set_initrd(char **dtb, off_t *dtb_size, off_t start, off_t end);
     ^

kexec/arch/mips/kexec-elf-mips.c:161:53: warning: passing argument 4 of
‘dtb_set_initrd’ makes integer from pointer without a cast
dtb_set_initrd(&dtb_buf, &dtb_length, initrd_buf, initrd_buf + initrd_size);
                                                     ^
In file included from kexec/arch/mips/kexec-elf-mips.c:33:0:
kexec/arch/mips/../../dt-ops.h:6:5: note: expected ‘off_t’ but argument is
of type ‘char *’
int dtb_set_initrd(char **dtb, off_t *dtb_size, off_t start, off_t end);
     ^

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>