powerpc/85xx: fix Kconfig warning about missing 8250 dependency

The SERIAL_8250_EXTENDED option just enables access to other
less regularly used options, like SERIAL_8250_SHARE_IRQ.
Select it to get rid of this warning when selecting the child
option living underneath it.

  warning: (FSL_SOC_BOOKE && SERIAL_8250_RM9K) selects
  SERIAL_8250_SHARE_IRQ which has unmet direct dependencies
  (HAS_IOMEM && SERIAL_8250_EXTENDED)

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>