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>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>