ARM: atmel: boards: use default CONFIG_SYS_PBSIZE

Entering the maximum number of characters defined by CONFIG_SYS_CBSIZE
into the console and hitting enter afterwards, causes a hang in the
system because CONFIG_SYS_PBSIZE is not capable of storing the extra
characters of the error message:
"Unknown command '' - try 'help'".

Use the default CONFIG_SYS_PBSIZE definition from config_fallbacks.h
to solve this problem.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>