include: sbi: Remove opensbi specific reset type defines

We can now use the standard SBI SRST extension reset types instead
of the opensbi specific (SBI_PLATFORM_RESET_xyz) reset types hence
remove related opensbi specific defines. The "platform_" prefix of
the reset type parameter of sbi_system_reset() function should also
be removed.

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