serial: samsung: no more support for S5P6440 and S5P6450 SoCs

This patch removes s5p64x0 related serial because of removing
support for s5p64x0 SoCs.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>