fsi: aspeed: Emit fewer barriers in opb operations

When setting up a read or write to the OPB memory space, we must perform
five or six AHB writes. The ordering of these up until the trigger write
does not matter, so use writel_relaxed.

The generated code goes from (Debian GCC 10.2.1-6):

        mov     r8, r3
        mcr     15, 0, sl, cr7, cr10, {4}
        str     sl, [r6, #20]
        mcr     15, 0, sl, cr7, cr10, {4}
        str     r3, [r6, #24]
        mcr     15, 0, sl, cr7, cr10, {4}
        str     r1, [r6, #28]
        mcr     15, 0, sl, cr7, cr10, {4}
        str     r2, [r6, #32]
        mcr     15, 0, sl, cr7, cr10, {4}
        mov     r1, #1
        str     r1, [r6, #64]   ; 0x40
        mcr     15, 0, sl, cr7, cr10, {4}
        str     r1, [r6, #4]

to this:

        str     r3, [r7, #20]
        str     r2, [r7, #24]
        str     r1, [r7, #28]
        str     r3, [r7, #64]
        mov     r8, #0
        mcr     15, 0, r8, cr7, cr10, {4}
        str     r3, [r7, #4]

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Tested-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20210223041737.171274-1-joel@jms.id.au


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