ti: keystone2: Move CONFIG_ISW_ENTRY_ADDR to a common place

ti: keystone2: Move CONFIG_ISW_ENTRY_ADDR to a common place

The ISW_ENTRY_ADDR Kconfig option under mach-omap2 isn't a SoC specific
notion but rather "where is our previous stage loaded in memory?"
option.  Make use of this on ARCH_KEYSTONE rather than SPL_TEXT_BASE for
our HS builds that are not using SPL anyhow.

Cc: Vitaly Andrianov <vitalya@ti.com>
Cc: Andrew F. Davis <afd@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com?