ARC: add hugetlb definitions

Add hugetlb definitions if THP enabled. ARC doesn't support
HugeTLB FS but it supports THP. Some kernel code such as pagemap
uses hugetlb definitions with THP.

This patch fixes ARC build issue (HPAGE_SIZE undeclared error) with
TRANSPARENT_HUGEPAGE enabled.

Signed-off-by: Pavel Kozlov <pavel.kozlov@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>