doc: Correct CPACR.FPEN usage

To avoid trapping from EL0/1, FPEN bits need to be set 0x3, not
clearing.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Change-Id: Ic34e9aeb876872883c5f040618ed6d50f21dacd0
1 file changed