restore p2m_access_t enum order to allow bitmask semantics

restore p2m_access_t enum order to allow bitmask semantics

Nested hap code assumed implict bitmask semantics of the p2m_access_t
enum prior to C/S 4c63692d7c38c5ac414fe97f8ef37b66e05abe5c

The change to the enum ordering broke this assumption and caused functional
problems for the nested hap code. As it may be error prone to audit and find
all other p2m_access users assuming bitmask semantics, instead restore the
previous enum order and make it explict that bitmask semantics are to be
preserved for the read, write and execute access types.

Signed-off-by: Malcolm Crossley <malcolm.crossley@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>