BL808: fatfs example didn't work on m0 core

[raspy135]

Tested board : Ox64

fatfs example didn't work on BL808 with m0 core.

The cause was too little .stack section (1kB) defined in bl808_flash_m0.ld.
There is a good amount of stack use for LFN support in ff.c.

Stack overflow will make random behavior.
Increasing stack size solved the issue.

Is the stack size set to 1kB for specific reason? My suggestion is increasing default stack size to 3kB for safety but I'm not sure it's good for the other examples. Since WiFi support is dropped, m0 might be able to use more memory?