clk: apn806: Add eMMC clock to system controller driver

Add fixed clock of 400MHz to system controller driver.  This clock is
used as SD/eMMC clock source.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-by: Omri Itach <omrii@marvell.com>
Reviewed-by: Hanna Hawa <hannah@marvell.com>
[fixed up conflicts, added error handling --rmk]
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>