arm64: Fix building error with 16KB pages and 36-bit VA

In such configuration, Linux uses only two pages of page tables and
__pud_populate() should not be used. However, the BUILD_BUG() triggers
since pud_sect() is still defined and the compiler cannot eliminate such
code, even though at run-time it should not be triggered. This patch
extends the #ifdef ARM64_64K_PAGES condition for pud_sect to include
PGTABLE_LEVELS < 3.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>