mach-davinci: Hard-code the default SPL_LDSCRIPT path

As there is only one linker script to use in this case, rather than use
the BOARDDIR variable hard-code the path.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
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Changes in v2.
- New patch