ARM: Add X600 board support (SPEAr600 based)

ARM: Add X600 board support (SPEAr600 based)

This patch adds support for the X600 SPEAr600 based board. Its also
the first SPEAr600 board that uses the newly introduced SPEAr600
SPL support. Xloader is not necessary any more. By using the new
"u-boot.spr" make target, one image will generated containing both,
U-Boot SPL (with mkimage header as needed by the SPEAr BootROM, and
the main U-Boot with mkimage header.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Amit Virdi <amit.virdi@st.com>
Cc: Vipin Kumar <vipin.kumar@st.com>