am33xx: Make SoC bootcount driver have its own symbol

am33xx: Make SoC bootcount driver have its own symbol

Some am33xx boards may not use the RTC block for bootcount (as it may
not be wired up for the board) and use some other facility.  So add
another symbol for the bootcount driver for the IP block.

Acked-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>