fbdev: Move framebuffer I/O helpers into <asm/fb.h>

Implement framebuffer I/O helpers, such as fb_read*() and fb_write*(),
in the architecture's <asm/fb.h> header file or the generic one.

The common case has been the use of regular I/O functions, such as
__raw_readb() or memset_io(). A few architectures used plain system-
memory reads and writes. Sparc used helpers for its SBus.

The architectures that used special cases provide the same code in
their __raw_*() I/O helpers. So the patch replaces this code with the
__raw_*() functions and moves it to <asm-generic/fb.h> for all
architectures.

v8:
	* remove garbage after commit-message tags
v6:
	* fix fb_readq()/fb_writeq() on 64-bit mips (kernel test robot)
v5:
	* include <linux/io.h> in <asm-generic/fb>; fix s390 build
v4:
	* ia64, loongarch, sparc64: add fb_mem*() to arch headers
	  to keep current semantics (Arnd)
v3:
	* implement all architectures with generic helpers
	* support reordering and native byte order (Geert, Arnd)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512102444.5438-7-tzimmermann@suse.de