Kconfig: Adds SYS_TEXT_BASE config option for Keystone2

This patch makes SYS_TEXT_BASE a config option for Keystone2
so that it can be used to load u-boot at different addresses
on secure and non-secure Keystone2 devices.

Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>