8xx serial, smc: add configurable SMC Rx buffer len

This patch adds the configuration option CONFIG_SYS_SMC_RXBUFLEN.
With this option it is possible to allow the receive
buffer for the SMC on 8xx to be greater then 1. In case
CONFIG_SYS_SMC_RXBUFLEN == 1 this driver works as the
old version.

When defining CONFIG_SYS_SMC_RXBUFLEN also
CONFIG_SYS_MAXIDLE must be defined to setup the maximum
idle timeout for the SMC.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/README b/README
index fcadf3f..56e981d 100644
--- a/README
+++ b/README
@@ -487,7 +487,7 @@
 - Console Rx buffer length
 		With CONFIG_SYS_SMC_RXBUFLEN it is possible to define
 		the maximum receive buffer length for the SMC.
-		This option is actual only for 82xx possible.
+		This option is actual only for 82xx and 8xx possible.
 		If using CONFIG_SYS_SMC_RXBUFLEN also CONFIG_SYS_MAXIDLE
 		must be defined, to setup the maximum idle timeout for
 		the SMC.