ARM: Page align secure section only when it is executed in situ

Targets that define CONFIG_ARMV7_SECURE_BASE will copy the secure section
to another address before execution.

Since the secure section in the u-boot image is only storage, there's
no reason to page align it and increase the binary image size.

Page align the secure section only when CONFIG_ARMV7_SECURE_BASE is not
defined. And instead of just aligning the __secure_start symbol, align
the whole .__secure_start section. This also makes the section empty,
so we need to add KEEP() to the input entry to prevent the section from
being garbage collected.

Also use ld constant "COMMONPAGESIZE" instead of hardcoded page size.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>