[ARM] remove bogus #ifdef CONFIG_HIGHMEM in show_pte()

The restriction on !CONFIG_HIGHMEM is unneeded since page tables are
currently never allocated with highmem pages, and actually disable PTE
dump whenever highmem is configured.  Let's have a dynamic test to better
describe the current limitation instead.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>