Change in coreboot[master]: device/mmio.h: Move readXp/writeXp helpers to device/mmio.h

[Hung-Te Lin (Code Review)]

Attention is currently required from: Arthur Heymans, Angel Pons, Jianjun Wang.
Hung-Te Lin has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/62561 )

Change subject: device/mmio.h: Move readXp/writeXp helpers to device/mmio.h
......................................................................

Patch Set 2:

(2 comments)

Patchset:

PS2: 
```
src/include/device/mmio.h: In function 'read64p':
src/include/device/mmio.h:227:16: error: implicit declaration of function 'read64'; did you mean 'read64p'? [-Werror=implicit-function-declaration]
```

Seems like read* and write* is also only defined in x86 arch mmio.

File src/arch/x86/include/arch/mmio.h:

https://review.coreboot.org/c/coreboot/+/62561/comment/85338a42_e148e66f 
PS2, Line 8: static __always_inline uint8_t read8(const volatile void *addr)
           : {
           : 	return *((volatile uint8_t *)(addr));
           : }
we may also need to move these into the top mmio.h.