spl: spl_legacy: Fix spl_end address

Currently, spl_end points to the __bss_end address, which
is an external RAM address instead of the end of the SPL text
section in the internal RAM.

This causes boot failures on imx6-colibri, for example:

```
Trying to boot from MMC1
SPL: Image overlaps SPL
resetting ...
```
Fix this problem by assigning spl_end to _image_binary_end, as this
symbol properly represents the end of the SPL text section.

From u-boot-spl.map:

.end
 *(.__end)
                0x00000000009121a4                _image_binary_end = .

Fixes: 77aed22b ("spl: spl_legacy: Add extra address checks")
Reported-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Marek Vasut <marex@denx.de> # DH i.MX6Q DHCOM PDK2