omap3evm: Use GENERATED_GBL_DATA_SIZE

The macro CONFIG_SYS_GBL_DATA_SIZE has been replaced
with GENERATED_GBL_DATA_SIZE.

Also define macros: CONFIG_SYS_INIT_RAM_ADDR and
CONFIG_SYS_INIT_RAM_SIZE.

Based on changes for omap3_beagle in the commit:
 31bfcf1c

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>