m68k: Remove ARCH= references from documentation

When building U-Boot we select the architecture via Kconfig and not ARCH
being passed in via the environment or make cmdline.

While in here, add the doc file to the MAINTAINERS entry for coldfire.

Cc: Huan Wang <alison.wang@nxp.com>
Cc: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Tom Rini <trini@konsulko.com>

MAINTAINERS

@@ -590,6 +590,7 @@ M:	Angelo Dureghello <angelo@sysam.it>
 S:	Maintained
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-coldfire.git
 F:	arch/m68k/
+F:	doc/arch/m68k.rst
 
 DFU
 M:	Lukasz Majewski <lukma@denx.de>

doc/arch/m68k.rst

@@ -35,7 +35,7 @@ development boards.
 Supported CPU families
 ----------------------
 
-Please "make menuconfig" with ARCH=m68k, or check arch/m68k/cpu to see the
+Please "make menuconfig" and select "m68k" or check arch/m68k/cpu to see the
 currently supported processor and families.
 
 
@@ -75,8 +75,8 @@ A bash script similar to the one below may be used:
    board=M5475DFE
 
    make distclean
-   make ARCH=m68k ${board}_defconfig
-   make ARCH=m68k KBUILD_VERBOSE=1
+   make ${board}_defconfig
+   make KBUILD_VERBOSE=1
 
 
 Adopted toolchains