vexpress64: remove #error

vexpress64: remove #error

This patch allows vexpress64 targets to be compiled when
CONFIG_SYS_FLASH_CFI is enabled.

I considered using #warning instead of #error, but this just clutters up
the build output and hides real warnings.

Without this patch, you see errors during compilation like this:

include/configs/vexpress_aemv8a.h:42:2: error: #error "Unknown board
variant"
 #error "Unknown board variant"
include/configs/vexpress_aemv8a.h:115:2: error: #error "Unknown board
variant"
 #error "Unknown board variant"
include/configs/vexpress_aemv8a.h:280:2: error: #error "Unknown board
variant"
 #error "Unknown board variant"
make[1]: *** [tools/envcrc.o] Error 1
make: *** [tools] Error 2
In file included from include/config.h:5:0,
                 from tools/envcrc.c:19:

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>