drivers/aspeed/common: Support disabled P2A bridge

This ports Linux commit 71f677a91046599ece96ebab21df956ce909c456
"Handle configuration without P2A bridge".

Quote:

    The ast driver configures a window to enable access into BMC
    memory space in order to read some configuration registers.

    If this window is disabled, which it can be from the BMC side,
    the ast driver can't function.

    Closing this window is a necessity for security if a machine's
    host side and BMC side are controlled by different parties;
    i.e. a cloud provider offering machines "bare metal".

P2A stands for primary to AHB.

Tested on Prodrive Hermes, which uses an AST2500. The machine still
boots, has a high resolution framebuffer working in EDK2, and its
boot time has been reduced by 2.5 seconds as it no longer runs into
a timeout due to disabled P2A bridge.

Change-Id: I3293dc35ae89c010154e02eff904ec3a68c96683
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45137
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

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