soc: aspeed: Improve kconfig

Reword the kconfig text to be consistent and reflect that most drivers
are available for all supported ASPEED chips (2400, 2500 and 2600).

Rearrange the symbols the SoC drivers depend on so the menu doesn't
appear unless you are building for ASPEED for compile testing.

The SYSCON_MFD and REGMAP options are usually selected by drivers that
need them, so do this.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200916125731.784527-1-joel@jms.id.au


Signed-off-by: Joel Stanley <joel@jms.id.au>