ARM: pxa: don't "select" SMC91X on MACH_XCEP

We normally don't hard-enable Kconfig options just because
a board contains a specific piece of hardware. In this case,
selecting SMC91X causes a build error, if we don't also enable
basic network device driver support.

Since the platform has no direct dependency on this driver
at link time, we can just remove the 'select' statement.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Daniel Mack <zonque@gmail.com>