iosys-map: Add a few more helpers

First the simplest ones:

	- iosys_map_memset(): when abstracting system and I/O memory,
	  just like the memcpy() use case, memset() also has dedicated
	  functions to be called for using IO memory.
	- iosys_map_memcpy_from(): we may need to copy data from I/O
	  memory, not only to.

In certain situations it's useful to be able to read or write to an
offset that is calculated by having the memory layout given by a struct
declaration. Usually we are going to read/write a u8, u16, u32 or u64.

As a pre-requisite for the implementation, add iosys_map_memcpy_from()
to be the equivalent of iosys_map_memcpy_to(), but in the other
direction. Then add 2 pairs of macros:

	- iosys_map_rd() / iosys_map_wr()
	- iosys_map_rd_field() / iosys_map_wr_field()

The first pair takes the C-type and offset to read/write. The second
pair uses a struct describing the layout of the mapping in order to
calculate the offset and size being read/written.

We could use readb, readw, readl, readq and the write* counterparts,
however due to alignment issues this may not work on all architectures.
If alignment needs to be checked to call the right function, it's not
possible to decide at compile-time which function to call: so just leave
the decision to the memcpy function that will do exactly that.

Finally, in order to use the above macros with a map derived from
another, add another initializer: IOSYS_MAP_INIT_OFFSET().

v2:
  - Rework IOSYS_MAP_INIT_OFFSET() so it doesn't rely on aliasing rules
    within the union
  - Add offset to both iosys_map_rd_field() and iosys_map_wr_field() to
    allow the struct itself to be at an offset from the mapping
  - Add documentation to iosys_map_rd_field() with example and expected
    memory layout
v3:
  - Drop kernel.h include as it's not needed anymore

Cc: Sumit Semwal <[email protected]>
Cc: Christian König <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Lucas De Marchi <[email protected]>
Reviewed-by: Mauro Carvalho Chehab <[email protected]>
Reviewed-by: Matt Atwood <[email protected]>
Reviewed-by: Thomas Zimmermann <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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