smp_call.h defines SMP-related structures and declares
SMP-related functions. These codes are only valid when
RT_USING_SMP is defined in the bsp, which means we have
to use RT_USING_SMP for judgment in every place where
this file is included, such as following:

```c
```

Now move the judgment of RT_USING_SMP directly into
smp_call.h, so that it's simpler to include the header
file as:

```c
```

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

For ARM, RT-smart and RT-standard both enable mmc (see
rt_hw_board_init() in bsp/cvitek/cv18xx_aarch64/board/board.c,
rt_hw_mmu_setup is called both for smart or standard).

So ioremap is must to do for ARM.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

Move the combine.sh to under cv18xx_aarch64, so we can
run this script the same as scons and no need to change
cwd.

Rename the folder boot to prebuild just becuase all the
files under boot are prebuild binaries.
Move the prebuilt fip.bin to under prebuild/milkv-duo256m
because it's only for 256.

Update combine.sh accordingly and output fip.bin to the output
folder where we create fip.bin/boot.sd as other bsp.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

Re-write README.md for aarch64.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>