powerpc: Remove unneccessary #ifdefs in reginfo

reginfo command is calling mpc8xx_reginfo(), mpc85xx_reginfo()
or mpc86xx_reginfo() based on CONFIG_ symbol.
As those 3 functions can't me defined at the same time, let's
rename them print_reginfo() to avoid the #ifdefs
The name is kept generic as it is not at all dependent on
powerpc arch and any other arch could want to also print
such information.

In addition, as the Makefile compiles cmd/reginfo.c only when
CONFIG_CMD_REGINFO is set, there is no need to enclose the U_BOOT_CMD
definition inside a #ifdef CONFIG_CMD_REGINFO

Lets all remove the #ifdefs around the U_BOOT_CMD as this
file is only compiled when CONFIG_CMD_REGINFO is defined

Finally, this is a PowerPC-only command, disable it on a number of
non-PowerPC platforms.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Tom Rini <trini@konsulko.com>