arc: minor fixes in Kconfig

arc: minor fixes in Kconfig

 [1] Fix misspeling in ARC_CACHE_LINE_SHIFT dependency, now cache-line
lenth selection is correctly enabled if either I$ or D$ are enabled.

 [2] Add dummy entry to target list to make sure target type is always
mentioned in defconfig. Otherwise defconfig for the first target in the
list will not have target name and later on with addition of the new
target on top of the list in Kconfig will lead to corrupted
configuration expanded from defconfig.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>