ARM: mvebu: Enable perf support in mvebu_v7_defconfig

Now that Armada 375/38x have support for the PMU, this commit enables perf
events in the defconfig.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>