serial: sh: Rename CONFIG_SCI and CONFIG_SCIF_USE_EXT_CLK to CFG_ variants

Both CONFIG_SCI and CONFIG_SCIF_USE_EXT_CLK options do not have a
matching Kconfig entry because they are internal to the SCIF driver.
Change their prefix to CFG_, i.e. CFG_SCIF_USE_EXT_CLK and CFG_SCI,
to reflect that and avoid interferring with Kconfig symbols. Since
neither of those options are defined elsewhere, no functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>