mx6_common: Fix LOADADDR and SYS_TEXT_BASE for MX6SL and MX6SX

Commit 81830581 ("imx6: centralise common boot options in
mx6_common.h") broke boot on mx6sl and mx6sx by assuming that all mx6
SoCs use the same LOADADDR/SYS_TEXT_BASE range, which is not correct.

DDR on mx6sx/mx6sl starts at 0x80000000.

Adjust LOADADDR/SYS_TEXT_BASE to the proper values for mx6sx/mx6sl,
so that these SoCs can boot again.

Also, TQMA6 requires a custom CONFIG_SYS_TEXT_BASE value, so move
its setting prior to the inclusion of mx6_common.h.

Signed-off-by: Fabio Estevam <fabio.estevam at freescale.com>