imx6slevk: fix uImage load address

i.MX6 SoloLite SoC has a different external memory base address
than the other i.MX6 family SoCs:
- 0x80000000 for i.MX6 SoloLite (see i.MX6 SoloLite Reference Manual [1])
- 0x10000000 for the others (see i.MX6Quad/Dual Reference Manual [2])

[1] http://cache.freescale.com/files/32bit/doc/ref_manual/IMX6SLRM.pdf?fasp=1
    Table 2-1 System Memory Map, MMDC - DDR Controller
[2] http://cache.freescale.com/files/32bit/doc/ref_manual/IMX6DQRM.pdf?fasp=1


    Table 2-1 System Memory Map, MMDC - DDR Controller

Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>