mx51evk: Use the 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: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>

include/configs/mx51evk.h

@@ -222,8 +222,6 @@
 #define CONFIG_SYS_HUSH_PARSER		/* use "hush" command parser */
 #define CONFIG_AUTO_COMPLETE
 #define CONFIG_SYS_CBSIZE		256	/* Console I/O Buffer Size */
-/* Print Buffer Size */
-#define CONFIG_SYS_PBSIZE (CONFIG_SYS_CBSIZE + sizeof(CONFIG_SYS_PROMPT) + 16)
 #define CONFIG_SYS_MAXARGS	16	/* max number of command args */
 #define CONFIG_SYS_BARGSIZE CONFIG_SYS_CBSIZE /* Boot Argument Buffer Size */