doc

This patch adds options for all the below mentioned configurations and
subsequently renames the include/configs/spearxxx.h files to spear3xx_evb.h,
spear6xx_evb.h etc to depict evaluation board configuration.

SPEAr3xx and SPEAr6xx boards can be compiled in following configurations
1. Environment placed in NAND
2. Console on usb device
3. Console on usb device with environment placed in NAND
4. SPEAr310 and SPEAr320 support environment variables in parallel
NOR flash.

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