vm-allocator: Add page size related functions

To avoid code duplication extract page related functions to their
own module and add utility functions for manipulating addresses
related to page sizes

Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>

vm-allocator/src/lib.rs

@@ -12,6 +12,8 @@
 
 mod address;
 mod gsi;
+/// page size related utility funtions
+pub mod page_size;
 mod system;
 
 pub use crate::address::AddressAllocator;