efi_loader: disable EFI_LOADER on vexpress_ca15_tc2 and ca9x4

efi_loader: disable EFI_LOADER on vexpress_ca15_tc2 and ca9x4

vexpress_ca15_tc2_defconfig and vexpress_ca9x4_defconfig do not provide
device trees (as $fdtcontroladdr) as needed to pass our Travis CI tests.

As we are on the move to requiring block devices to be based on the driver
model these boards either will have to be converted to use device trees or
will be kicked out of U-Boot.

So let's disable CONFIG_EFI_LOADER on these boards until the device tree
issue is resolved.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>