ARM: at91: Enable SPL_SEPARATE_BSS by default

According to the linker script for both armv7 and arm926ejs based SoC,
BSS section was all the time separated for SPL but this symbol was
only enabled on some boards. However, it is necessary to have it
enabled for OF_SEPARATE configuration where DTB is appended to u-boot
with DTB.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>