ARM: tegra: refactor tegra{20,30}_boot_secondary

"tegra_boot_secondary()" has many condition branches for some Tegra
SoC generations in a single function so that it's not easy to compile
a kernel only for a single SoC if one wants with some reason, debug
purpose(?). This patch provides SoC specific version of
boot_secondary(), tegra{20,30}_boot_secondary(). This could allow
any combination of SoC to be built. Those boot_secondary functions can
be preparation when we ntroduce chip specific function pointers in the
future without having chip dependent branches around.

Also removed unused definition/prototpye.

Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
[josephl: remove the Tegra114 part of the original patch]
Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>