ARM: fix arch/arm/Makefile for Tegra

ARM: fix arch/arm/Makefile for Tegra

Since commit 79d75d752717 (ARM: move -march=* and -mtune= options to
arch/arm/Makefile), all the Tegra boards are broken because the SPL
is built for ARMv7.

Insert Tegra-specific code to arch/arm/Makefile to set compiler
flags for an earlier ARM architecture.

Note:
The v1 patch for commit 79d75d752717 *was* correct when it was
submitted.  Notice it was originally written for multi .config
configuration where Kconfig set CONFIG_CPU_V7/CONFIG_CPU_ARM720T for
Tegra U-Boot Main/SPL, respectively.  But, until it was merged into
the mainline, commit e02ee2548afe (kconfig: switch to single .config
configuration) had been already applied there.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Stephen Warren <swarren@nvidia.com>
Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Jan Kiszka <jan.kiszka@siemens.com>