tegra: kconfig: move board select menu and common settings

Becuase the board select menu in arch/arm/Kconfig is too big,
move the Tegra board select menu to tegra/Kconfig.

Insert the Tegra SoC select menu between the arch select and the
board select.

 Architecture select
 |-- Tegra Platform (Tegra)
     |- Tegra SoC select (Tegra20 / 30 / 114 / 124)
        |- Board select

Consolidate also common settings (CONFIG_SYS_CPU="armv7" and
CONFIG_SYS_SOC="tegra*") and always "select" CONFIG_SPL as follows:

  config TEGRA
          bool
          select SPL

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Cc: Tom Warren <twarren@nvidia.com>