vexpress64: increase max gunzip size

vexpress64 kernels are usually over 8 MBytes in length, so setting the
max uImage length to 64 Mbytes should give us plenty of scope for
expansion.

I mostly chose this length to match other board configs that use
"(64 << 20)".

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