drivers: ti_qspi: use syscon to get the address ctrl_mod_mmap register

We used to get the address of the optionnal ctrl_mod_mmap register as the
third memory range of the "reg" property. the linux driver moved to use a
syscon instead. In order to keep the DTS as close as possible to that of
linux, we move to using a syscon as well.

If SYSCON is not supported, the driver reverts to the old way of getting
the address from the 3rd memory range

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>