spl: Kconfig: Escape '$(ARCH)' in LDSCRIPT entries

The default SPL / TPL linker script is in the $(ARCH) directory.  The
way we use this today works but isn't ideal.  With an update to Kconfig
to re-sync with the Linux Kernel, we need to escape the '$' here so that
it will end up being evaluated by make.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>