include: Drop MMIO from SBI_PLATFORM_HAS_MMIO_TIMER_VALUE

It is not necessary that platform has MMIO-based timer value
register. It can also have some custom (implementation specific)
CSR for timer value.

This patch renames SBI_PLATFORM_HAS_MMIO_TIMER_VALUE to
SBI_PLATFORM_HAS_TIMER_VALUE to imply "platform timer value"
instead of "mmio timer value".

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