zynqmp: add "override" directive to mandatory options

zynqmp: add "override" directive to mandatory options

The platform.mk sets build options required for ZynqMP, but users
can still change them from the command line, like:

  make PLAT=zynqmp RESET_TO_BL31=0 CROSS_COMPILE=...

Then, the makefile shows an error message in that case:
  Using BL31 as the reset vector is only one option supported on ZynqMP.
  Please set RESET_TO_BL31 to 1.

If the option is not user-configurable, the makefile can specify
"override" to prevent users from changing it.  We do not need the
error message for the case that never happens.

Likewise, ENABLE_PLAT_COMPAT := 0 and PROGRAMMABLE_RESET_ADDRESS := 1
are mandatory to avoid build error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Soren Brinkmann <soren.brinkmann@xilinx.com>