km_arm: change CONFIG_SYS_TEXT_BASE to end of RAM

This allows to test a larger part of the RAM in the memory tests.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>