ARM: AM43xx: Protect against jump to bad return address from rtc-only

For am43xx_rtconly_evm_defconfig, the bootloader indicates support for
RTC-Only modes by writing a magic number into RTC scratch register
which kernel PM code will then clear and then write back again if
RTC-Only mode entry is desired. If no PM code is loaded in the kernel,
this magic number will not get cleared and then be detected again
by the bootloader on a warm reboot, indicating an RTC+DDR resume event
and causing the bootloader to attempt to jump to the resume address in
RTC scratch register 0, which is invalid.

Add a check to prevent jumping to address 0 if the scratch register is
not properly programmed, otherwise warm reboot will not work on am437x
if no PM is loaded in the kernel.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>