Prior to this commit, esp_psram/include/esp_private/mmu.h contains some
APIs that is used for:
- copy flash content to psram
- necessary sync APIs used by flash_mmap.c, due to above feature
This commit rename it to mmu_psram_flash.h, therefore mmu.h can be used
for real mmu related APIs.
This commit also moves above mention funcitons in `mmu.c` and
`mmu_psram.c` to `mmu_psram_flash.c`, leaving `mmu.c` to be used for
real mmu driver.
