Makefile: Set ABI, ISA and Code Model in top-level make

This patch introduces following optional PLATFORM options:
PLATFORM_RISCV_XLEN -> RISC-V register width
PLATFORM_RISCV_ABI -> RISC-V GCC ABI
PLATFORM_RISCV_ISA -> RISC-V GCC ISA string
PLATFORM_RISCV_CODE_MODEL -> RISC-V GCC Code Model

If the above options are not provided by platform config.mk
or by command-line parameters then:
1. PLATFORM_RISCV_XLEN will be determined using toolchain
capability
2. PLATFORM_RISCV_ABI, PLATFORM_RISCV_ISA, and
PLATFORM_RISCV_CODE_MODEL is set to value best suited for
generic libsbi.a

As a result of these optional PLATFORM options, the
platform-cflags-y and platform-asflags-y is further
simplified for platform config.mk.

Signed-off-by: Anup Patel <anup.patel@wdc.com>