ARM: vexpress_*_defconfig: replace earlyprintk with earlycon

earlyprintk no longer works on arm64 platforms. Replace it with earlycon
which works fine.

Cc: Ryan Harkin <ryan.harkin@linaro.org>
Cc: Liviu Dudau <liviu.dudau@foss.arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>