libxc: arm: use INVALID_P2M_ENTRY

libxc: arm: use INVALID_P2M_ENTRY

Albeit INVALID_P2M_ENTRY and INVALID_MFN are both ~0UL, conceptually
speaking we should use INVALID_P2M_ENTRY for setting P2M values.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>