rtc: Add Xicor/Intersil X1205 RTC support

This patch adds support for the Xicor/Intersil X1205 RTC used on the
AMCC Makalu eval board. This driver is basically cloned from the Linux
driver version (2.6.23).

This patch also introduces the Linux bcd.h header for the BCD2BIN/
BIN2BCD conversions. In the future some of the other U-Boot RTC driver
should be converted to also use this header instead of implementing
their own local copy of these functions/macros.

Signed-off-by: Stefan Roese <sr@denx.de>