arm: spear: fix enabling of SSP2 clock

The SSP2 clock is at bit 6 in the register, so the value is 0x40 unlike
the current 0x70 which enables the clock of UART2, SSP1 and SSP2.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Acked-by: Stefan Roese <sr@denx.de>