multiboot2: Replace u_phys with u32
    
    u_phys is used just in two places and sometimes it may confuse reader.
    Additionally, GRUB multiboot2 implementation does not use u_phys anywhere.
    So, replace it with basic well defined and used in implementation u32 type.
    
    Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
    Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>