SPEAr: Correct the definition of CONFIG_SYS_MONITOR_BASE

The below text is copy pasted from README
- CONFIG_SYS_MONITOR_BASE:
	Physical start address of boot monitor code (set by
	make config files to be same as the text base address
	(TEXT_BASE) used when linking) - same as
	CONFIG_SYS_FLASH_BASE when booting from flash.

This patch corrects the definition of CONFIG_SYS_MONITOR_BASE and sets it to
TEXT_BASE

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>