ARM: move -march=* and -mtune= options to arch/arm/Makefile

My main motivations for this commit are:

[1] Follow the arch/arm/Makefile style of Linux Kernel

[2] Maintain compiler options systematically
  Currently, we give -march=* and -mtune=* options inconsistently:
  Only some of the CPUs pass -march=* and -mtune=* options.
  By collecting such options into the single place arch/arm/Makefile
  we can tell which options are missing at a glance.

[3] Prepare for deprecating arch/*/cpu/*/config.mk

Note:
  This commit just moves the compiler options so as not to change
  the behavior at all.  It does not care about the correctness of
  the given options.  Fox example, "-march=armv5te" might be better
  than "-march=armv4" for ARM946EJS, but it is beyond the scope this
  commit.  Also, filling the missing -march=* and -tune=* is left
  to follow-up patches.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Stefan Roese <sr@denx.de>

arch/arm/cpu/pxa/config.mk

@@ -6,8 +6,6 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-PLATFORM_CPPFLAGS += -mcpu=xscale
-
 #
 # !WARNING!
 # The PXA's OneNAND SPL uses .text.0 and .text.1 segments to allow booting from