soc: aspeed: Re-enable FWH2AHB on AST2600

soc: aspeed: Re-enable FWH2AHB on AST2600

Recent builds of the vendor u-boot tree disable features of the
BMC that may allow unwanted access if not correctly configured. This
includes the firmware hub to ahb bridge (FWH2AHB), which is used by this
driver.

The bit to "un-disable" it is in the SCU. Set it only when the ioctl is
called and we are running on the ast2600, as to not open up the
'backdoor' unless there's userspace trying to use it.

Fixes: deb50313ba83 ("soc: aspeed-lpc-ctrl: LPC to AHB mapping on ast2600")
Link: https://lore.kernel.org/r/20210629073520.318514-2-joel@jms.id.au
Signed-off-by: Joel Stanley <joel@jms.id.au>