arm64: add an option to switch visibility of CONFIG_SYS_INIT_SP_BSS_OFFSET

arm64: add an option to switch visibility of CONFIG_SYS_INIT_SP_BSS_OFFSET

By default, CONFIG_SYS_INIT_SP_BSS_OFFSET was made invisible by not
giving a prompt to it.

The only way to define it is to hard-code an extra entry in SoC/board
Kconfig, like arch/arm/mach-tegra/tegra{186,210}/Kconfig.

Add a prompt to it in order to allow defconfig files to specify the
value of CONFIG_SYS_INIT_SP_BSS_OFFSET.

With this, CONFIG_SYS_INIT_SP_BSS_OFFSET would become always visible.
So, we need a new bool option to turn it off by default.

I move the 'default 524288' to the common place. This value is not too
big, but is big enough to avoid the overwrap of DT in most platforms.
If 512KB is not a suitable choice for your platform, you can change it
from your defconfig or menuconfig etc.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Stephen Warren <swarren@nvidia.com>