DOC: update xip method info

today:

- dcssblk: s390 dcss block device driver

An address space operation named get_xip_page is used to retrieve reference

to a struct page. To address the target page, a reference to an address_space,

and a sector number is provided. A 3rd argument indicates whether the

function should allocate blocks if needed.

An address space operation named get_xip_mem is used to retrieve references

to a page frame number and a kernel address. To obtain these values a reference

to an address_space is provided. This function assigns values to the kmem and

pfn parameters. The third argument indicates whether the function should allocate

blocks if needed.

This address space operation is mutually exclusive with readpage&writepage that

do page cache read/write operations.