am43xx: Do not allow EMIF to control DDR_RESET in rtconly config

Prevent EMIF control of DDR_RESET line on DDR3 am43xx platforms for
am43xx_evm_rtconly_config. Without this DDR is unstable and can become
corrupted after multiple iterations of RTC+DDR mode.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
[j-keerthy@ti.com Ported to latest master branch]
Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>