lib/rsa: Kconfig: Remove superfluous 'depends on RSA'

RSA_SOFTWARE_EXP and RSA_FREESCALE_EXP are wrapped inside:

if RSA
	...
endif

So, remove the redundant "depends on RSA" from their depends expression.
In addition, move SPL_RSA into the same "if RSA ... endif" block, since
its only direct dependeny is CONFIG_RSA. This tidies up and simplifies
reading of lib/rsa/Kconfig.

No functional change intended.

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Simon Glass <sjg@chromium.org>