platform: Identify correct bmc platform based on bmc hw version

At the moment the generic platform sets bmc_generic() as bmc platform
which does not have any support to initialize the flash and hence it
fails to load petitboot kernel.

[  583.105000325,4] FLASH: Failed to load VERSION data
[  583.105490257,5] INIT: Waiting for kernel...
[  583.105523156,5] INIT: platform wait for kernel load failed
[  583.105555219,5] INIT: Assuming kernel at 0x20000000
[  583.105589925,3] INIT: ELF header not found. Assuming raw binary.
[...]
[  583.299682673,5] INIT: Starting kernel at 0x20000000, fdt at 0x30a44eb0
1274673 bytes
[  583.344432417,3] ***********************************************
[  583.344490230,3] Fatal Exception 0x800 at 0000000020000000
MSR 9000000000000000
[  583.344535875,3] CFAR : 0000000030022948 MSR  : 9000000000000000
[  583.344578019,3] SRR0 : 0000000020000000 SRR1 : 9000000000000000
[  583.344620242,3] HSRR0: 0000000020000000 HSRR1: 9000000000000000

OPAL builds the device tree for BMC based system using HDAT. It
populates bmc/compatible node with bmc hw version e.g.
"ibm,ast2600,openbmc". Use that to identify proper BMC hw board and
initialize BMC platform with proper backend. This allows opal to
successfully load and boot into petitboot kernel.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Reza Arbab <arbab@linux.ibm.com>