firmware: fw_base: Introduce optional fw_platform_init()

We add optional fw_platform_init() function which will allow
platform specific code to update "struct sbi_platform platform"
before it is used.

The fw_platform_init() can be a regular C function so before
callint it we:
1. zero-out BSS section
2. Setup temporary trap handler to catch bugs
3. Setup temporary stack pointer

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>