arm64: kvm: Introduce nvhe stack size constants

arm64: kvm: Introduce nvhe stack size constants

Refactor nvhe stack code to use NVHE_STACK_SIZE/SHIFT constants,
instead of directly using PAGE_SIZE/SHIFT. This makes the code a bit
easier to read, without introducing any functional changes.

Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Link: https://lore.kernel.org/r/20241112003336.1375584-1-kaleshsingh@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>