spl: Use SPL_TEXT_BASE instead of ISW_ENTRY_ADDR

The ISW_ENTRY_ADDR symbol was used for OMAP devices in place of
SPL_TEXT_BASE. Keystone2 HS devices were not using it right either.
Remove ISW_ENTRY_ADDR and use SPL_TEXT_BASE directly.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>