i386: Don't include <grub/cpu/linux.h> in coreboot and ieee1275 startup.S

i386: Don't include <grub/cpu/linux.h> in coreboot and ieee1275 startup.S

Nothing defined in the header file is used in the assembly code but it
may lead to build errors if some headers are included through this and
contains definitions that are not recognized by the assembler, e.g.:

../include/grub/types.h: Assembler messages:
../include/grub/types.h:76: Error: no such instruction: `typedef signed char grub_int8_t'
../include/grub/types.h:77: Error: no such instruction: `typedef short grub_int16_t'
../include/grub/types.h:78: Error: no such instruction: `typedef int grub_int32_t'

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>