ARM: tegra: align carveout size

Align the size of the carveout region to 2M. This ensures that the size
can be accurately represented by an LPAE page table that uses sections.

This solves a bug (hang at boot time soon after printing the DRAM size)
that only shows up when the following two commits are merged together:
d32e86bd ARM: HYP/non-sec: enable ARMV7_LPAE if HYP mode is supported
6e584e63 ARM: tegra: avoid using secure carveout RAM

Cc: Mark Kettenis <kettenis@openbsd.org>
Cc: Alexander Graf <agraf@suse.de>
Acked-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>