reset: sti: simplify driver's config and build

reset: sti: simplify driver's config and build

With commit 64933513e461 ("reset: sti: Remove STiH415/6 reset support"),
there is only one sti reset driver left and there no need to define a
dedicated config STI_RESET_SYSCFG, which is selected by the config for the
STiH407 reset driver.

Simply add reset-syscfg.c to the build for the STiH407 reset driver.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://lore.kernel.org/r/20230330112347.31137-2-lukas.bulwahn@gmail.com
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>