arm: omap: delete unincluded omap-common/config.mk

arch/arm/cpu/armv7/omap-common/config.mk is never included
because "omap-common" is not SoC name.

If we want to add OMAP-specific compiler flags,
they must be added to omap3/config.mk, omap4/config.mk, omap5/config.mk.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>

arch/arm/cpu/armv7/omap-common/config.mk

-#
-# (C) Copyright 2002
-# Gary Jennejohn, DENX Software Engineering, <garyj@denx.de>
-#
-# SPDX-License-Identifier:	GPL-2.0+
-#
-
-# Make ARMv5 to allow more compilers to work, even though its v7a.
-PLATFORM_CPPFLAGS += -march=armv5