configs: Replace CONFIG_ISW_ENTRY_ADDR with CONFIG_SYS_TEXT_BASE

Since 7e0ed13f ("Convert ARCH_OMAP2PLUS boards' CONFIG_SYS_TEXT_BASE to
Kconfig"), a default SYS_TEXT_BASE was set for all ARCH_OMAP2PLUS devices.
CONFIG_ISW_ENTRY_ADDR is used to set SYS_TEXT_BASE in qspi boot.

Simplify this by directly assigning SYS_TEXT_BASE in the defconfig.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>