fix(doc): match boot-order size to implementation

Docs had boot-order field as being u32 but code uses uint16_t.
FF-A specification does not specify a required size.

Signed-off-by: Kathleen Capella <kathleen.capella@arm.com>
Change-Id: Ia4c3fc66b9e733ea1417d34c2601bce1f81c4d32
1 file changed