PCI: brcmstb: Add bcm7278 PERST# support

PCI: brcmstb: Add bcm7278 PERST# support

The PERST# bit was moved to a different register in 7278-type STB chips.
In addition, the polarity of the bit was also changed; for other chips
writing a 1 specified assert; for 7278-type chips, writing a 0 specifies
assert.  Of course, PERST# is a PCIe asserted-low signal.

While we are here, also change the bridge_sw_init_set() functions so like
the perst_set() functions they are chip specific and we no longer rely on
data wrt chip specific field mask and shift values.

Link: https://lore.kernel.org/r/20200911175232.19016-6-james.quinlan@broadcom.com
Signed-off-by: Jim Quinlan <jquinlan@broadcom.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>